I really don't see a problem with it flying. It may work out great. You may want to consider moving the firewall back even more to help with the CG problems the gas engine will create. Did you put your servos in the tail? If not, this is a must. The battery may even have to go back there.

I am currently finishing up a Goldberg Ultimate that will have a Homelite 25cc on it. I got the CG in the range with that short fuselage, so being that the stinger is longer, you should be able to do it.

By the way, I currently fly a 60 sized Stik with a 25cc Homelite on it and it is awesome. Unlimited vertical and a great sound.

Look into ways to hop-up the engine by searching some of the forums around this site. I got upwards of 800 rpm improvement from my homie by researching the Internet and following advice.

Can you remove the fins on the flywheel of that engine? It will reduce weight and reduce air drag, just be sure you have a way to balance it.

Hope some of this nonsense helps you!

Tom