The tanks get stained from the inside out. There's basically nothing you'll be able to do about it except paint it, which isn't a good idea on a rider. Keep an eye out for a good one. My KX had an unstained tank on it when I got it but the PO put too long of a bolt in the petcock and broke it and a couple of the inserts spun when we took the mounting brackets off. The tank I ended up with polished up nice but it's stained and darker green than it should be. The other '81 KX I'm building has a beautiful unstained green tank on it, I don't understand why some stain bad and others don't stain at all. I would figure it had to have gas left in it at some point in it's 32 year life.
I knew that thing had to have an Iron Cross on it somewhere, they just can't help themselves.
Are you going to Vintage Days at Mid-Ohio this weekend? You might be able to find a pipe there, and may even come up with a nice tank. It's kind of new but I bet there'll be parts there for it.
Anyway, the bike looks great, nice save.
KRH