Clear coat it with some polyurethane or urethane that is made for autos or other outdoor vehicles. I haven't been too happy with the rattle can clear coats I've bought at the big box building supply stores that claim they are exterior. I didn't use them on a bicycle though. However they didn't seem to last long for the lawn art and other things my wife likes to buy.

Or just clean it up and keep a coat of car wax on it.... if they make car wax today. Been several decades since I've used it.

I like the character and history such stuff relates too.