Probably the best upgrade I did to my 80's bike was putting new wheels on it. That had its own problems too as modern wheels don't come with freewheel hubs, also requiring an upgrade fron a 6sp freewheel to a 9sp cassette. Luckily, it was a steel frame that could be expanded to accept it. (Actually, it had already expanded as I was having to compress the rear stays to get them to drop on the wheel prior to the upgrade.