Yep... heat. I "hot-knifed" a stab off of an old airframe just the other day... Between the heat gun and hot knife they can do alot with cured epoxy. I like the "soldering iron knife" idea.. If I had a ton of joints to work on, I'd for sure go that route. For the hot-xacto use a propane torch if you can... I actually just used a candle I had handy, which worked fine, but it does leave a little carbon to clean up.. no big deal.

Your plane being an ARF actually didn't sustain too much damage... It looks to be a pretty clean break... and very fixable. Good luck.