1990 Yamaha 90 -> yamaha 5 leg

I Have a 1990 Yamaha 90hp on my inflatable, the motor goes well but i want more speed!!!
I have tried many different props, unhooking the rev limiter, loosing as much weight as possible etc.
by my calculations this machine needs approx 115hp but......no more weight.
I was considering matching a yamaha 50hp leg to the 90hp powerhead because it is lighter with a smaller profile in the water.
I realise the gearbox ratios are different and thus i will re-prop etc.
Is anyone out there as crazy as me? Has anyone tried this? Will the 50 water pump provide enough cooling water?
cheers

What is the highest tach reading at W.O.T.? If your engine is making the high end of W.O.T, like 6000 rpm, try a higher pitched prop. Do not mess around putting different gearcases on the engine, you may just blow it up. Try lifting the engine higher on the transom. Watch for overheat.