Why stop at 50hp? Find an older mariner 60hp 2cylinder 2 stroke they weigh like 160#, I am sure you can find a good used 70 or 90hp motor you can shoe horn on that transom... All of what I said to this point is a joke.
Seriously that rating is on there for a reason, if you need more speed buy a boat that can handle it. No short cuts here. I once looked at buying an over powered boat, before I made an offer I called my states boat commission and asked about an over powered boat. Thay told me I would get a ticket is ever checked, and with what every one has already said it all points to BAD NEWS. What will you gain? couple MPH.