The weight of the 65 FS and 70 FS is an ounce or so more than the Irvine 53 so weight should not be a big issue other than needing add about ½ ounce more weight to the tail. My MonoKote ARF Kaos needed quite a bit of weight in the tail to balance with the Irvine 53 and OS 46. My Kaos kit balances perfectly with the Irvine 53 or OS 56 and is about ½ lb lighter than the ARF. The 70 FS will be a tight fit and getting it to mount back far enough could be a challenge. The Irvine 53 has about 1.7 HP and pulls much stronger than the OS 46. The Irvine moves the kit very well so a 70 FS in the ARF my not be enough power. I tried mounting one of my RJL K.61s in the ARF and the fuselage front was too narrow for it to fit.