If you're trying to do a restoration to showroom condition, then NOS or new but identical levers should be used. The fenders could be rechromed but doing it right is very difficult and expensive, if you can get used parts that are in excellent condition it will probably be cheaper. The biggest problem is usually the rivets that hold the assembly together; it has to be disassembled for rechroming but those rivets are hard to reproduce properly. Having the individual parts stripped, pits filled, polished, and rechromed should be done by a specialist restoration shop and they do not come cheap.
Doing a concours restoration on a 550 K3 is probably not a wise investment. The 750, 350, 400 are somewhat "cult" bikes and a super low mileage untitled bike - particularly a year 1 specimen - would get an astronomical price if sold properly. The 550, though, doesn't have the cachet or value of the others.
