The gas tank shouldn't be a problem (if its an external tank). Correct me if I'm wrong someone. Try cleaning it first. If not, you can probably get a gas tank from another moped (even if not a Motomarina) that might fit. Lot's of mopeds used what I think are somewhat "universal" parts. Check out the photo gallery on this site, you might find some mopeds similar in body style to yours.
Did you say the engine itself is a Motomarina? It's not a Minarelli or a Morini engine? If so, a picture would definitely be something people would like to see. Where was this bike made (Italy, US, somewhere else)? Is there a stamp on the frame somewhere?