I just posted the same "shift adjustment" question this weekend. My 2 speed shifts too early. One fella said if the ATF level is wrong, this would affect it. Mine's ok however. It's supposed to be up to the side-plug level. So I still don't know what I can do to adjust mine.
Concerning the tank rust, there are lots of posts in the archive. They cover the spectrum of advice on this. I've used the Kreem kit of acid/acetone/sealer-paint on a motorcycle tank, and it worked very well. But it was alot of work, and I clogged the 2 ports at the bottom lobes of the tank w/ the paint, which was a hassle. On my Puch tank, I wanted to forgoe the effort and cost of the Kreem kit, so I tried Naval jelly (any hardware store) diluted w/ 25% water. I left it in for 2 hours, and it did almost nothing. If you go that route, plan on overnight-2-3 days. If I was going to do it all again, I'd spend the $30 on the Kreem Kit and do it right.