It will fit... and using the 720's rev limiter... it will probably make that extra 5 hp... but what you don't see in the specs is a torque difference. If your pump is in good shape, the 650 will have a hard time pushing the pump to it's peak RPM's
But... the short answer is... yes it will work, and since you already have the engine... it's a cheap way to get out on the water.
Oh... almost forgot... you will need to use the 650's CDI/coil. The 720 may work, but I'm unsure of the timing.
Good luck