Re: Sport Jet 90 Powerhead to 120
There is a solas impeller you can buy, but it will get you 1 to 2 mph best case. You can also fill in the wear ring with jbweld, this is probably your best option.
It comes down to to the fact that engine never really was impressive to begin with. Old technology (force based, not merc), and a fairly bad jet pump design coupled to it. 39-40 mph would be an extreme case and probably with a good tail wind.
I don't know if anyone has done it, but one cheap mod you COULD do is either have the heads milled down, or use a thinner head gasket. This would up your compression ratio, but also require you to run premium gas. Could be good for maybe 5 to 10 hp. The problem with a jet is that horsepower by itself is almost meaningless, without modifying the pump to take advantage of that. (see above, same reason putting a 120 into a 90 pump won't do much of anything) Jet pumps don't care about torque, the ONLY thing that matters is that the engine can hit the operating rpm with the load given by the pump.
As someone else mentioned, trim tabs will make a world of difference, but you still won't get more power, it just helps it plane much easier.