Welcome to the place.
You said that there were some mods done to the unit, have these been checked or removed?
The amount of signal that is recycled to the record circuit can be limited by so many things that you will need to systematically check each part of the record and playback circuits (both electronic and mechanical) to try and optimize the circuit. A failing coupling cap or a misaligned head may cause similar audio symptoms.
As for the motor, if you have basic mechanical skills and are very careful about disassembly and reassembly cleaning and servicing the bearings and the oil felts should not be a problem. Remember that there will be a raised burr on the main shafts of the motor caused by the fan and pulley set screws. You must remove these burrs before trying to slide the shaft through the bearings or you will damage them.
Have you checked the rubber idler wheel for roundness and surface condition? I find that most wow and flutter relate to the idler wheel and capstan rather than the motor. The motor rotation is reduced down so much by the pulley and drive belt that unless there is a major problem with the motor, little variation gets to the actual capstan drive shaft.