Considering where you are on the project, this is what I would do.  Find a way to estimate the weight if engine required for best balance.  That will give you a clue.  I agree with flamin that the 120-4stroke is likely too lo in power.  Ags are supposed to be hi powered planes.  If you can stand the weight, a G38 or G45 would be a nice match for the airframe.  If lower weight is called for there are a bunch of light weight gassers available in the same size class.

Looks like fun.

Bedford