As you know with the water prop on a G23 they do take the mods well, but they start to get there power around the 13,000rpm mark and max out somewhere around 17,000 to 18,000rpm's, the air props are designed to work from around 10,500 rpm's to 12,000 rpm's just below where the G23 needs to be. At the same time your boat will run ok with the G23, but much better with a G26. I have tried a few different engines, brands and sizes, so for the best engine I have found for the gas airboats is the CY 27cc. The G26 ran very close to the CY so if you can find a G26 at a good deal go with it.
The ideal size for the G23 IMO 38" x 18" x 2-1/2" the radio box would need to be 3" tall to have room for the 1/4 scale servo.
You have a good start on your boat now and it will run ok like it is.