Mark,
Marantz cassette decks are notorious for failure with drive drive belt problems. I have known of many folks who had to put their decks down because of it. These decks, as with most other decks are all belt driven internally, and the belts get stretched and worn. If you are real insistent on finding one of these beauties, you may have to consider sourcing some new belts, or find someone that can service them and bring them back to life.
I just got my old Sony ES deck going and it was an adjustment on pinch roller tension. This deck is also belt driven and at some point will be SOL and will also have to be put down. I take it that you are building an all Marantz system, which sure is fun, but also has some roadblocks to over come.
Good hunting.
Wayner