Hello guys, I am student in college and bike is something pretty new to me.

Anyhow, I just got a 1980 Fuji Monterey and I found that the freewheel is really rusty. I don't know whether I should try to replace it if it's possible. It is a 5 speed but I'm not clear about the size. If I shouldn't, or don't have to change it, is there any way to clean it out?

I also appreciate any other advice on the bike.