It might be too late but take pictures of EVERYTHING, they'll come in handy when reassembling.
The first time working on a cassette player is a little intimidating, just be patient.
Clean any dust, dirt or gunky grease off of the mechanisms, wheels, etc.
If they haven't been changed already, I'd replace the belts while you're in there.
If you do, make sure to clean the belt paths with alcohol.
Also, clean any rubber tires and the places they contact in the player with some alcohol so they're not slipping because of dirt/dust/grease.
I always check the condition of the pinch roller.
Out of the 25+ boxes that I've refurbed, I only had to change one or two pinch rollers.
Remove it and make sure it's not cracked and still fairly soft.
If the surface is really dirty, you can lightly run it on some 180/150 grit sandpaper.
I wash them with a toothbrush, cream hand cleaner (Goop, Gojo) and then Dawn dish soap and let them dry.
Afterwards they look almost new.
Taking the capstan out and cleaning and reoiling with synthetic oil is also a good idea.
Just make sure not to lose any springs or tiny washers that might be on the shaft.
Make sure to clean the oil off of the shaft before reinstalling the clean pinch roller.
You don't want to get oil on the pinch roller or the tape as it's running between them.
You lucked out with the gears for these being readily available.
