Can you put an 80 horse outboard on a boat rated at 65 max or will you have to re enforse the transom?

The strength of the transom isn't the problem. There's a fairly complex formula to determine the biggest engine your boat can safely use - it takes into account the length of the boat and width of the transom. If you put more engine on the boat than it's rated for, it could cause an accident you'd be held responsible for.