There are many reasons why the deck might play slower than it should that might not be related to the motor. First you got new belts. Were they too short? too thick, too stiff? All that can contribute. Yes, I know you might have ordered the "correct length". But factory original OEM M90 belts aren't manufactured anymore. All we can get is generic "compatible" ones and the quality as well as sizing are always a bit off, as well as the quality. Then there's grease that may have dried up and adds friction to the rotating assembly. Then there's corrosion that could have built up increasing friction. Then there's the capstan backlash. Did you adjust that or check it? Also did you pull out the capstan needle and check to see if there's any corrosion on the shaft? I always polish that shaft and add a tiny drop of silicone grease or a drop of oil there. Then there's circuitry that supplies power to the motor. There is a regulator that supplies current to that circuit (X703 if I remember right) and also a fusible resistor too. Fusible resistors are notorious for changing values over time because they run so hot, and if it has, then it could also change the amount of current to the motor, possibly causing the voltage to droop during use. In any event, when the potentiometer runs out of adjustment, then it's time to evaluate the other potential causes, because when the adjustment is up against the stop, any further twist will just twist that tiny potentiometer cap off as the shell is probably as thin as aluminum foil.
As for replacing the motor, yes, boombox enthusiasts have probably all replaced cassette motors at one time or another. Almost all boombox capstan motors are very similar and they almost all have the exact same mounting pattern. You'll find that in general, they all have 6 mounting holes on the face which allows you to "clock" the motor so that the rear wire connections face the original way. Finding a new factory original replacement is going to be virtually impossible as those haven't been manufactured for ages. And due to their value, everyone who owns an M90, even ones that are in horrible shape thinks that their unit is a restoration candidate when in reality, they're probably a better candidate to be parted out. In other words, hardly anyone is willing to part out an M90 anymore, they're always joining the ever growing group of M90 owners searching for parts. Getting a compatible motor is probably key to getting the deck working again. And you'd be wise to check all of the fusible resistors too to ensure that their values are still within tolerance.
I'm sending you a PM shortly. Check it.
