If they have never flown this plane, then engine recommendations only sound good on paper. Some planes dont always work according to the manual. This plane was designed around a Irvine two stroke, it shouldnt be sooo tailheavy unless you installed servos or batteries in bad spots, or put heavy pushrods in the tail...
You might can get away with keeping your 160 up front by moving your engine forward on the mount as far as it will go, or moving your batteries, receiver etc. up front as far as you can. Heck, even adding another battery and switch for redundncy will add some weight up front... A slightly tailheavy plane is not a terrible thing.
It sounds like you've flown it already without major problems like losing control of the plane, so it sounds to me like it cant be that bad. I always move my CG back further than what the manual says, its just the way I fly.
If its flyable as it sits right now, put your batteries in the nose, fly it, and see if you like it.. Just move stuff around, or add weight untill you like the way it flies.
Good luck
