[Electronic M]

The disc with the brown edge and white pulley should be the rotor of the capstan motor that regulates tape speed.

That white pulley should drive an orange-ish toothed belt on the underside that goes to a pulley that drives the mech for the cassette tape reels. I need more pictures of the whole mech to tell you more.

I last worked on my Sony super beta a good 3-5 years ago so memory is foggy. I wish I could get a capstan rotor for mine...I had to replace a bad capacitor on the capstan motor board and burned a hole with my soldering iron in the brown motor magnetic control track on the side of the rotor and it has not tracked right since (it did cure the won't stay in play mode issue I had though).

If it loads a tape properly, wraps tape around the drum, but does not pull the tape back in to the cassette after unwrapping the drum before ejecting the tape then I'd disregard what Findem-Keepem said.