Hi Dave
This is a very broad question!
Most Märklin decoders require a DC motor (magnet inductor)
So on old Märklin, the motor may have to be converted to DC using one the the Märklin motors: 60941 (drum collector), 60943 (small flat collector), 60944 (large flat collector)

On analogue locos it depends if Märklin or not:
- Märklin locos used to have a mechanical direction relay which can be replaced by a decoder (Märklin manages to keep the same dimensions
- for non- Märklin locos you have to check if there is enough space to fit a decoder. A mLD3 Märklin decodeur measures 16 x 5 x 11.5 mm (version with cables (no plug-in PCB)

Note: The above is assuming that the loco is already in 3 rails (electrically linked axles and with a slider to pick power from studs)

Converting a loco is delicate work requiring an electronic soldering iron (small soldering pad, regulated temperature) electronic solder wire, heath shrink (1 mm diameter). ALWAYS, prior to ANY conversion, make sure the loco runs fine and smooth (good pick-up, good motor).
The conversion may include replacing bulbs with LEDs and installing a sound decoder (more space is required and it makes only sense on a high value and SILENT loco).
I find it fun to do. Together with creating a sound project may take 2 to 3 hours.

Cheers
Jean

With the correct skill, and tools, it's fun.

This is a good idea if you wish to run old locos on a new digital layout ”as they were back in the days”.

However it’s a no go if you wish to upgrade and have the really smooth digital running.

This idea is alsol most likely the easiest way to do the conversion. However it’s not really that much cheaper than a new 5-pole DC motor and a mLD3 decoder.

But a good point anyway 🙌

-Eino

Thanks guys for all your comments. By the sound of it, I'll leave well alone LOL