They're all right, 50hp is overkill and aside from the excess wait and no appreciable gain in performance the boat is built light for it's day compared to it's contemporaries. Expect to have to reinforce the transom to accommodate the weight if you go big hp. It's a simple boat and a good one to learn on. The mast is a plain extrusion with cast aluminum shoe fitted at bottom. Depending on how bad the crack is consider getting it welded. We used the same casting on a bunch of masts we built for Grampian competitors so maybe you find a replacement on a similar damaged mast in a nearby yard. Fits into the deck step with bolt across the step and through the casting. You might be able to swing it down but you will need a couple of friends to avoid dropping it, or check threads on SNet about the subject.
We used to call these grunts in the boatyard (we had names for all of 'em) but a whole bunch were built proving they were popular.
Where are you located? Might be able to direct you if you are in Ontario.