Impellor!
Usually, when we want to pump up our motor's you think of re-working the engine with high performance parts. A re-build would let you change the piston stroke and valves, etc, .....but with the Rotax engine, I don't know if that's even possible. I'm sure that it probably is...
I think the first and probably the least expensive would be to find an impellor with a different pitch. I'm sure there are aftermarket shops that sell this stuff. I think I even saw an ad on our website who primarly dealt in pumping up your PWC.......checkum out!...