I have a 3W-100 (7lb version) in my 33% 540-T. I don't think the lighter engines will do you much good if you plan to put the 4 servos in the tail. Mine balances without added lead. The only problem with the 3W ignitions is if the plug caps start leaking to ground. This was a problem with the old style caps. This was fixed over a year ago on the new engines shipped out. You can also retrofit the older ignitions with the new style 3W cap or DA cap. It costs $20/cap.

If you like to be different, you could also stick a ZDZ-120 twin (about 7 lbs) in the plane. You would save yourself about $250 and have more power than any 100cc motor.