I was using Symoba close coupler mechanisms for cars ( http://www.symoba-schniering.de/ ), replacing original couplers and super glue the mechanism. There are several pocket lengths available. I bought my stuff from Axel Schnug, friendly and dependable ( http://www.schnug-modellbahn.de/index.html ).

---

In a series of MM articles, Axel Schnug describes also the retrofitting of earlier locomotive models with close couplings:
https://www.maerklin.de/...emen-specials-und-serien
Search for: "Alte Modelle - kurz gekuppelt (MM 04/2002 bis 04/2006)".

Inter alia, you searched for Märklin 3054, the 103 electric loco.
Axel Schnug's MM article "Alte Modelle kurz gekuppelt", episode 2, MM 5/2002, page 50 describes the conversion of such an engine. Link: http://medienpdb.maerkli...ls/Kurz_gekuppelt_02.pdf

Translation of the German description (translated with DeepL.com (free version)):

"Conversion of the 103 series
Although the work on the 110 and 140 locomotives is pleasingly simple, the BR 103 locomotive, on the other hand, unfortunately does not make it easy for the prospective rebuilder. Although both the matching bogie frames (440640 in black) and the required drawbar (440630) are available as spare parts, if you fit these to an older locomotive, the installation height of the coupling will no longer fit. A connection with wagons is then not possible. The reason for this is the modified motor bogie and the bogie frame: The illustration shows the difference. Three conversion options were examined: The bogie frame with the drawbar 404020, on which the springs were cut off, can be seen on the left in the picture (Fig. below). Although the reduced coupling distance has a positive visual effect, the coupling sits too low and therefore does not couple well on all wagons. The example on the right shows the conversion with the old frame and a 344890 drawbar fitted. To do this, the springs on the frame have to be cut off. The locomotive thus couples very tightly, but the coupling height is also too low, which is particularly annoying when changing locomotives frequently; nevertheless, this is a labor-saving and cost-effective alternative. In the center of the picture is the bogie frame 440640, which has been adjusted to the required support dimension using a grinding wheel. This conversion requires a little patience, as it is necessary to repeatedly install and remove the frame to check the height during the grinding process. As the frame and drawbars have to be procured from scratch in this case, this method is more expensive than the other two solutions, but it undoubtedly leads to the best result because it both minimizes the coupling distance and leads to the correct height. If a particularly close vehicle connection is required, the drawbar 344890 intended for the 110/140 can also be installed. However, this requires an extensive solo test run of the intended train set in all conceivable directions before it is put into regular service on your own layout! The downer, however, is that in this case the stability of the newly fitted coupling does not match that of the old coupling hook.[...]
Axel Schnug, Holzminden"

Then, you mentioned the Märklin 3021, the V200 diesel. I don't know if this applies by analogy, but Axel Schnug reports on the conversion of a class 216 diesel - scroll down to last picture at the bottom of the page.
http://www.schnug-modell...bauten/adapter671860.htm

Translation of the German description (translated with DeepL.com (free version)):

"Conversion tip for locomotives with a pivot point: adapter 671860
Conversion of locomotives with a pivot point mount to NEM shaft
Some Märklin locomotives have a coupler pocket with a pivot point. Either a close coupling or a Relex coupling made of plastic with a metal bracket and front uncoupling is fitted here ex works. Often, however, you will only find a simplified coupling without a pre-decoupling function or even just a coupling hook. In any case, this coupling is held in the middle position with 2 springs. On many of these locomotives, the old coupling can be replaced by the adapter 671860 and a coupling head according to NEM. The advantages:
Shortening of the coupling distance
Free choice of coupling head
Option to install a current-carrying coupling.
The adapter can be fitted to these two locomotives without any problems. However, the return springs must be at the bottom when inserting the adapter. If the adapter is inserted the wrong way round, the spring effect is not effective.
However, the adapter does not fit in all locomotives. The BR 515 battery-powered railcars, for example, can only be converted with an exchange coupling from the 7205 pack. I supply this coupling separately under the number 263730. The adapter cannot be fitted to very old models either because the coupling mount is too narrow. This mainly affects class 151 and 216 locomotives. Alternatively, a replacement bogie frame could of course be installed here. The photos below show how to distinguish between the individual types:
[...]
Class 216
Locomotives of this series can usually be optimized with 671860. Very old versions such as this 3075 still have a metal plate on the coupling bracket of the bogie frame. In this case, the frame must be replaced. The individual part numbers are 469620 for the bogie with slider and 393990 for the drive bogie shown. "

I can happily recommend a German seller/manufacturer with the name Rudolf Wansorra who sells very inexpensive adapters under the brand "Mobadapter". He is very popular among the users of the German "Stummi forum". I bought a pair of his adapters specifically available for the Märklin 3021. It is very easy to attach to the loco and reduces the distance to the following car. He also has adapters for many other models. Well worth a try before starting to make major changes to the locomotive.

See this Ebay auction: https://www.ebay.de/itm/285001982870

It depends on what you are running behind your 103. If you are running the old 24cm TEE coaches, those couplers are hard to replace.
If you are running with the initial 27cm coaches (4095-4099), those coaches had extended coupling lengths. The batch from the mid-1980s had a little shorter coupling arm.
The 4295-4298 27cm models had close couplings. Using these coaches, even with an older E103/E03 would reduce the space between the locomotive and the coaches.

I don't remember what the mounting for the coupler on the 3054 was. If you can find somebody who makes a replacement NEM socket, Hag in Switzerland made different length coupler heads (no loop). I have used the shortest one (I think it was intended for the 152/154 BDe 4/4) with my Maerklin Traxx Hobby loks to reduce the space between those models and the first car.

For the 3021, I don't remember seeing the spacing being as big as it was with the 3054.

Regards

Mike C

To all that responded, thank you for your help.

I will sift through all the info and links you sent me.

Yes, for the 3054 I will be pulling the 24 cm cars, but I was not going to change the couplers on the cars, just the engine.

I will also explore other ideas, which may include 3d printing. Yes, I need to find out how much pull weight are 10 cars (and only on flat track, no ramps) before I will attempt a 3d printed coupler.

Thank you again.

Leo P