Nakamichi CR-7A Does not fast forword or rewind

Bcr,
I can't give you the technical reasons that you're having problems with your Nak but will just say that there is more going on under the hood of your machine than just a few belts
and pulleys. Typically, people buy used machines and think that they can update them easily with parts bought on Ebay. I
just had my RX-505 re-worked by a local tech (Willy Hermann).
It wasn't cheap but it sounds and works like new. Get some
professional help and I think you will be glad you did. I also
have a CR-7A and can tell you that it is worth the investment.

Tonkay's right. There's way too much under the bonnet and will require specialist help.
To me, it sounds like the dreaded "orange capacitor disease".
There was a period when Nakamichi used a particular brand of capacitor, (and yes, they were orange in colour), only to find out that some time down the track, they failed.
To localise which particular capacitor it is, you would have to refer to the service manual relative to the FF and RW section, then go through the caps one by one to find the culprit/s.
Not an easy job, and best suited to someone familiar with the ins and outs of these things.

Sorry.....