Biggest problem would be getting the sender unit in the tank. You would have to cut a hole, then worry about sealing it. You could find a sender off an old ski (say a Waverunner III), or try one of the aftermarket units from an auto-parts store. The gauge itself would be cheap.
Might be easier to swap out an entire tank from a different ski that already is set up for a gauge (like the III) if it would fit. May be more trouble than its worth.