Well, it's looking like the photointerruptor is OK, so that's something.
If the capstan motor is literally a 2-wire DC motor and not some multi-pole thing, it might be worth testing continuity on the windings. You'd have to unplug it from the deck for that, though to avoid getting false readings. Not sure what the resistance of a motor that size is likely to be, but you should get something. Might also be worth turning the motor gently while taking the measurements in case the commutator is doing something funny to the circuit in its current position.
I would have thought that if the motor's really getting the full 35V or so, it should be turning. That might indicate a problem with the motor itself, which might be fixable, e.g. a problem with the brushes. Alternatively I could be getting confused and we're still looking at a problem with the cut-out, e.g. the power transistor driving the motor has gone bad.
However, this really is guesswork, to be honest I'm out of my depth here.
OK. The motor pins appear to be 3 and 4 on the J2 connector.
Other points of interest are U809, which is the 2SC3400 switching transistor that appears to be responsible for effecting the cutoff switch. Q803, which is also associated with that, and finally Q5. Q5 is on a PCB all of its own and is marked as safety-critical so my guess is that that's the power transistor which is actually driving the capstan motor. P801 is the connector for it. Pin 2 on the 801 connector corresponds to pin 4 on J2. Pin 3 on 801 is ground.
My guess is that when the the tension arm is lifted, you should be getting something like 35V on J2-pin 4 with respect to ground. If that's not the case, it might be worth checking that you get something like that between pins 2 and 3 on the 801 connector for the power transistor.
It's late, I'm stretching my knowledge of electronics and I'm not sure if this will be coherent when I read it back in the morning, but it's the best I can do. Hope it's some use :3