Wow! A half pound in the tail is a LOT for that size airplane! Even though you're putting in a 108, that would still be lighter than a 120 4-stroke. I would double check that you've got the position right on your CG - and if you do I'd do everything possible to shift weight back (Even if it meant making a new battery pack where the cells were loose so you could slide the batteries in one at a time, or 3 behind the servos and one in front etc)

Maybe even see if you can slide the engine closer to the firewall