It is almost impossible to offer any practical advice without pictures but Bash is spot on. The engine
is going to weigh 150+ pounds and I suspect is worth a few boat bucks.
So 150# for the shell, 150# for the engine and you are already at 300# - The engine is going to load the transom pretty well. I would be thinking at least 3/4 inch and perhaps 1 inch marine
ply glassed in for the transom with the load carried into the sides of the boat.
Then perhaps two bulkheads incorporated into rear and forward benches to add rigidity to the shell with perhaps a small foredeck triangle.
I know this sounds like way more work than you want to do and it wil likely end up way heavier than 500#.
Depending on use of the boat have you considered a smaller motor
? A lot easier if you are talking 9hp.