It hasn't rusted much so it's not likely to deteriorate. It's not a piece of Jap Crap. If the repairs work leave the frame alone. It's part of the bike's history. DO NOT take the bike apart until you are ready to fix it. There are loads of restored 16Hs. There are far fewer with patina. If it were mine I'd do whatever was needed to get it running again but otherwise leave it alone where ever possible