The good thing about that particular engine is that it is a great platform to modify. The 61x engine (701cc, single carb, 63hp) has great bottom end. There are a ton of aftermarket upgrades for that engine if you decide down the road that you will need more power. Now as far as your current power to boat ratio, you may find it a little under powered. It all really depends on how your boat is dressed out once the build is complete. For this to be a good performer, you will need to be weight conscience. One of my prior builds was a 14/36 with the yamaha 650cc 50hp. It did ok and could reach 30mph but most of us cruise around 25 and for that boat to keep up it was running **** near wide open and drinking a lot of fuel. We just upgraded to a 760cc 90hp and now it cruises 25 to 30 at half throttle and will hit 40 on top end. The extra power really helps on the fuel economy. So, your build will perform at a decent level but you may find that you are really in the throttle most of the time and the engine will be grunting. Also, yes to your last question, any Yamaha from the 760cc down will bolt into the same motor mounts. You may have to switch out couplers as we found out on our 650cc to 760cc swap but everything else lines right up.
