Looking good so far. I see these get flown with a RCGF 10cc quite often and it seems to give pretty good performance. You have to use two piece mounts because the rear carburetor can be annoying/impossible to fit within a radial mount a lot of the time.
What 9cc do you have in mind? I had a NGH 9cc I got off hobbyking because they had an insane sale before they stopped selling them. I never flew it, but did bench run it a bit and it seemed to run pretty well. It has a front intake and many glow mufflers will fit on it (I think it has the same bolt pattern as the OS 55AX) so it'd be a lot easier to install and minimize cutting up the firewall and cowl cheeks. NGH advertises it as a 40-46 glow engine replacement, so you should be fine in terms of power. Contrary to popular belief,
they are still manufactured and you can still buy them.
If you don't mind spending extra money you could also get a Saito FG14. It's based on the FA-82 and factoring in the reduced power, would probably give you somewhere around 70-sized glow 4 cycle performance, which is entirely adequate. A 4 stroke would also fit a large low speed airplane such as this quite a bit better....