Hey Bredgeo,
BEFORE you do anything about your motor, you really should focus on the much more common and likely issues:
(1) Belts worn, stretched, or slipping. I know you said you replaced them 2 years ago but they could be slipping, especially if they were contaminated during the install process which is easy to do when slipping the belts over parts with grease, fingers etc.
(2) There are 4 fusible resistors in the cassette logic control board. It is very very common for them to blow or change values as they age due to heat -- and they DO generate lots of heat. IF they have changed values, especially if higher value, the motor will NOT get full voltage and it will run slow or drag.
(3) Remember that the M90 cassette deck has 2 motors. A capstan motor and a FF/RW motor.
BTW, partstore.com is notorious for showing classic/antique parts on their website. They will gladly take your order, then inform you it's on back order, then back order again, then cancel your order 6 or 8 weeks later. I have thus far NEVER received anything from them except 1 service manual. About a dozen other orders were always accepted but never fulfilled. Their database is woefully out of date.
PM me your mailing address. I will send you a FULL set of belts. I will also send you instructions on how to remove/change your belts and locations to test your fusible resistors.
Norm. :super: