Sounds like you could easily put a 1.20 glow engine, or a 20cc gas rig on it... maybe a tad bigger if you have some experience with them... With that said, I might even go to a 30cc gas engine... That would definitely give your plane some serious power... I would say the safest bet would be a 20cc gasser, and you would see a substantial increase in power. There's several glow engines in the 1.20 range in the classifieds lately as well... a 1.20 glow engine would also increase performance drastically... It depends on your skill level and how you want to approach it though.
I have the Great Planes Stik 60, it has a 66.5" span. I have an OS .95 four-stroke and it's a fire-breather... prior to that I had a 1.10 four-stroke, and it was incredible... so.. .yeah.. you need a bigger engine for sure. Good luck.