How crusty is the inside of the tank? For about $50.00, you can buy a product called Caswell, an epoxy tank sealent. It basically seals any rust in. If it's really bad, then forget it. If it's mostly surface rust with no heavy coating, you can just do a simple wash and use it. Use it on my DT racer and has been a great asset.
"The bold adventurer succeeds the best.” - Ovid
WTB: Plated KTM EXC450/525 RFS, Honda CRF450X, Suzuki RMZ450