Hey pal,

I would strongly recommend disconnecting the cable before changing the flat rear. You could either disconnect at the barrel adjuster that's along the chainstay (first choice), or unscrew the indicator chain from the hub.

Looks like you have a bit of clean-up to do, but there's definitely good potential. I have a 1974 that's the same color. It's a size or two smaller, and was my uncle's. If you want any help with getting yours spiffed up, I should have time to help after the Holidays.