My GMS .76 has a higher stated ( if you can believe the vendors! ) bHP rating than the ST G.90.

I'm running it with the rather small 11x7P prop, so you should be fine.

You will need to redo the engine mount blocks, but this is not a big problem.

Also note that deep under the fiberglass, is a VERY hard wooden mounting base ( NP did this right ), so as long as you use washers or larger blind nuts you should not have a problem.

You may need to add a larger battery pack to the plane at the nose to offset the weight of the motor.

I'd imagine that a 1800 to 2400mAh 6v pack will more than do the job.

The SuperTigre muffler is fairly blunt on the tail end ( which will end up facing forward ) so this will add a good bit of drag.