I'm afraid it will be hard to replace the motor. There was a deceased gentleman who did make replacement motors - ModelTorque
You might ask for it via the website to see "if" you can get one 2nd hand.
Google Lima replacement motors perhaps to see what other may have done.
Otherwise it will be a MAJOR operation worth more than the loco and needing some skill. You would have to pull the loco apart. Motor , removed, and even getting one main driving wheel out, not damaging the valvegear. Then buying a motor-gearboxunit and a new axle and main gear to fit to the lima one. Then redo the loco with the gearbox/motor somehow.
There is also the issue og wheelstandards, the Lima loco will not work on scale track - code 83 or 75. Again no easy way to regrind the wheels down.
The few things you can do is to clean the motor, oil and greese the mechanism clean the wheels, perhaps add a resister to the loco to limit full speed. One can fit a DCC decoder easy enough as well.
I hope others have something different to add !