I would start by seeing how much rust you have,
if it is just rusty water and no big flakes, remove, empty, flush, swish some new oil around for a coating and away you go after you replace all your filters and clean the carb.
If you have big flakes coming out, its time to replace.
rob
How difficult is the tank to remove? If you can get it out there is a product called Kreem thats used to recoat the inside of tanks. Used on old motorcycles all the time.
