Some models had the coupler attached to an arm which was part of the bogie frame. Others had a separate part which was attached to the pivot point of the bogie.
You could possibly try to replace some of the bogies with spare parts from Ritter Restaurationen or other source.
You could also perhaps find a used model of the same type that has bogies in better shape than the ones on your current models, which might cost less than the parts themselves.
It may be possible to kitbash a plastic coupling shaft and NEM coupler from a Roco model, providing there is space between the bogie and the undercarriage. You would also have to figure out some kind of fixation that would keep it in place.

One last option would be to look at coupling mechanisms by companies like Symoba(symoba-schniering.de) which might be adapted for the older coaches. You would have to adapt the bogie so that the coupler would no longer be part of the bogie assembly.

Regards

Mike C

Madhu, I don't know what availability is like in India but here in the UK (where Marklin is not the most common) the easiest and cheapest thing to do is to buy cheap wagons etc off Ebay and strip the couplers off them. Small coaches and tipper wagons that were originally in the cheapest starter sets are best (if they fit!).

As a bonus you'll end up with a lot of wheels too...

Michael

There is always a way... But you'll still be using ice cream sticks in any case :)

You might want to try a few things first:

1) I wrote a blurb about this type of rolling stock a few months ago here. I run some similar age and condition cars by the sound of what you are saying. I get by with the 7001, even with rusty couplings.

One thing to check (and not in the instruction manual) is the decoupling latch orientation (the bit that gets caught in the central contacts), once the coupling is level and fits snuggly in the 7001 gauge, the decoupling latch needs to be checked and adjusted if needed to give enough clearance for the central contacts. I use a few m-track 5106 attached to a 5128 crossing as a base when I adjust my couplings, this way i can check I have proper clearance in the worst case scenario.

Another thing to check is that the spring attached to hooks on both couplings on some type of cars is present. If it isn't, no amount of adjusting will give you the correct height.

2) Using couplings of different types together is always a source of problems based on my experience, so I run trains made of cars equipped with similar couplings (basically metal only or plastic only), and have an easy life :) Changing all your couplings might help a bit, but you might not get an even height across your fleet, so you will face the same problems, or cause new problems (hint: derailments).

So I would think twice before doing anything drastic: changing a coupling can be vey easy: remove screw, remove coupling, insert new coupling, tighten screw. It might require more complicated steps: find and buy the right part, dismantle car, taking care of not damaging 50 year old plastic/paint/metal, bend this, bend that, remove coupling or boogie, same again in reverse order. After all that, you'll find that you are unable to adjust the height of a given type of coupling or that plastic couplings/NEM pockets break from time to time and need replacing.

How much money and time you want to throw at the problem is your business. For me it would not be worth the effort, I would invest a bit more time experimenting with the good old 7001 and a bit of money in a nice new thin pair of pliers...