Wow, you really did disassemble it quite far, which was something that even the most seasoned and experienced boombox enthusiast would typically refrain from attempting. Generally speaking, a simple rebelting and cleaning, and selective lubrication is sufficient to restore these to fully working condition. I would say, only disassemble as far as necessary to accomplish those tasks. There is no real benefit to disassemble parts any further because the risk of losing the small bits, breaking parts, etc is far too great and aside from the belts, virtually nothing else is available anymore.
At this point, here are a few recommendations:
1) If you are sure that you haven't lost any parts, it is certainly possible to reassemble everything. However, you are now at the point where you probably need the guidance of a service manual. If it was my boombox, I wouldn't even attempt it without the aid of a exploded view schematic of the mechanical section, which you can find in the service manual. A copy can be purchased here:
PDF service manual for Panasonic RX-5100, RX-5100A
2) If you think you've gone too far and don't mind spending a little bit more to get it back to where it was, it may be worthwhile to consider picking up a parts unit to salvage the cassette deck from. There are a bunch of them right now on eBay ranging from $48 to $150 plus shipping. No it's not free, but if your unit has sentimental value, and you want to restore "your" unit, then this is an easy(er) way of doing it. You'd still have to rebelt it, but there's really no need for all the keys, etc to have been extracted, as those are totally unrelated to the drive asembly. $48 plus shipping is not free or cheap, but how much is your time and sanity worth?
3) And if you think you may be picking up another unit instead, and you aren't married to that particular model, you may wish to consider upgrading to the RX-5150 or RX-5250 models. The price range of those are significantly higher but you'll be getting a better boombox too. You might be able to sell your unit off for parts or as a complete fixer to help subsidize the other purchase.
Good luck.
