It's difficult (if not illegal) to buy a small 2 stroke in CA these days and under the assumption your tank was for gas it could be used. However the built in tank may have a capacity greater than needed or practical. It's also probably pretty old and you'd want to overhaul it well to clean, check for leakage, venting etc. Might be better with a smaller portable tank above decks.
Check with some local Honda dealers to see what they would charge to install a 10 HP. That will give you an idea of the high end of prices for the job.