E mail Modellbahn Union in Germany and ask for a retrofit motor. They sell many. Any can motor that fits any Marklin br 216, or 218 will also fit.

This loco has a Soft Sine motor, which is why it has the flexible flat cable. A normal motor is not a suitable replacement without changing the electronics.

Why do you believe the motor is faulty?

It is more likely to be the electronics on that model, and replacement electronics are available from SB Modellbau I understand. I haven't had occaision to try replacing any of these, but other people seem quite pleased with them.

Hi Thor
The motor is a SDS, usually the best motor Märklin ever produced but it fails sometimes.
I understand one good way to make them fail is to lubricate them. Have you? They have ball bearings.

Those SDS motor are not available since a decade or so (result of Märklin's bankruptcy.
Please doublecheck that the SDS motor or its driving PCB (not the 21MTC decoder) as converting to a "normal" motor is a huge jump backwards (but sometimes you have no choice)

For different reasons I replaced the 21MTC decoder by a mSD3 on a SNCF CC40108 and it works perfectly. I did the programming with a CS3 and nothing else except a memory stick for storing sounds

Clean all surplus of oil and grease and lubricate with mârklin oil (very very little) and Trix grease on the wormgears.
Wipe any dirt, oil on the SDS motor and do not lubricate it.
I have 10 SDS locos and their running is fabulous compared with today's 600€ locos
Cheers
Jean

PS : those SDS motors do not like voltage fluctuations as they convert into speed fluctuations so make sure pick-up is good and rails, wheels, slider are clean

I would not assume anything. Often it can be the earthing through the wheels etc… this causing the same power supply issues as a worn/dirty slider.

Novelty is a word Märklin often use to describe something they are selling that year.

I don't understand it either.
Fact is that the SDS was rather small.
When Märklin switched to dirt-cheap cost-effective can motors, they had to adjust the loco frame to make more room for the motor.
I think it is very unlikely that there will be smaller motors to replace the SDS in the future. Since I have quite a few SDS locos, I'd be happy to know there are replacements.