If the rear derailleur is working (and it likely is), I'd leave it alone. The plastic/delrin front derailleurs (and yours has been replaced) have a bad reputation but the rears tend to work. I'd ride it before thinking about replacing it.

If you want/need to fiddle with the rear derailleur, this blog post will give you a few options:

https://www.redclovercomponents.com/blog/1

You could track down a set of 120 OLD 700c alloy wheels to get that bike on the road. The easiest solution may be to learn how to deal with tubulars. They ride great but they can be a pain to deal with.

You did a great job of cleaning up the bike but you will want to overhaul it and replace the consumables before doing much riding on it.