To clarify its a SeaArk MVJT 1872 that this mod was done to and it required a significant amount of time with a boat builder and very talented aluminum welder. A much stouter hull than the g3, that required transom and stringer rebuild/design. The engine was a Honda BF150>105 at the jet.
Really when scoping a jet like that you aren’t seeking top end, theres literally only so much water the jet impeller can suck and this limits you from coming unglued (like you see with way over powered outboards with props). I should add running a tunnel hull jet at any speed over 23-25 knots is not comfortable or really what that particular set up is meant to do.
It’s a significant undertaking that requires the time and attention of a certified builder. I totally agree that one should not just willy nilly slap larger engines on boats that the hull isnt rated for. I am also a strong advocate of putting the maximum hp that the boat is rated for, makes life easier. But what the OP is describing is a very bad idea, in all seriousness he should flip the engine and use the proceeds to buy a Honda or Yamaha 70………..if you can find one.