They aren't the best on gas, but I'm sure you expected that.
It might still be a crossflow engine, and the loop charged are much stronger engines. I would try and search that out. Loop charged-much stronger power band, and more parts available.
Looks like a looper cowling and pans to me. Give me a model number off of the port side transom bracket and I will tell you for sure. If so there is no way that is a 150hp engine. That was never produced in that configuration. The lowest horsepower in either the 165 C.I or the 183 C.I. was 200hp and was conservative at that.
Thats a loop charged 200hp Johnson...er according to the cowl, hence GT200. Whats the tag say on it? I don't know enough about johnnyrudes to know if you can swap the cowls around like that and it doesn't look pieced together anyway.