motorcycle strap
I have a M 26 S with a 8 horse tohatsu, not at all in the same weight catagory as your 50 hp, but I have been using a strap with a cam buckle. One end around the motor shaft, the other connected to the stern rail. Cinching up on the cam buckle helps support the motor. I got the strap at Wal-mart, it's advertised purpose a tydown strap for motorcycles. I haven't had my boat very long, but so far the transom and rail seem to be holding up.