I'd have to disagree with you there Terbo......

The .20 would stand a good chance of tearing that little hull apart, as well as not being able to keep it on the water.

The G-Mark .06 should move that little hull to around 15mph. If you want more power, but still be able to control it, I'd move up to an .09 or .10. That in itself would make this size hull a little water rocket. I generally limit hull size with an .10 to around 18"-22" (shortest to longest length).
The beam at 8" to my experience MAY BE a little narrow, but would be workable.

If I remember right, this is a balsa hulled boat ?

IMO and experience with these small airboats, a .20 is too much power. Those dimensions are better suited for 1/2a to .10 size engines.