Getting fuel out of balsa is pretty easy actually. Go to your local pharmacy and find an aerosol spray called K2R - it sprays on white and absorbs the fuel from the wood - you'll have to clean out the white stuff when it dries, and be very careful not to inhale this stuff when you're spraying/cleaning it out. Had to do it for my sister's ARF trainer when the fuel tank stopper cracked and leaked all over the insides.

Cheers