So I took the bottom cover off this unit today and sure enough the big belt is broken and in fact is hard as a rock. I also checked out the Sony cassette deck and all four belts are bad on that as well and that one's going to be a joy to clean up. I found one place that had all the belts I needed
https://www.electronicsupplycorp.com/
This is not an official recommendation as I haven't actually ordered yet, just sent an email telling them what I needed (5 minutes ago). However the prices are reasonable. Thank you to @redk9258 for the catalog, that was super helpful.
Now one question about this unit. Two of the belts look to be replaceable simply by cleaning the pulleys and gently installing the new ones. However there's one (that isn't broken on my unit) that looks like it requires some disassembly to replace, any advice? I have not been able to find a service manual for this unit.
For "Loading Belt 1" any ideas? Or am I just going to have to play with it?
Thanks again guys your help has been very valuable.