The Homelite 25s are better for scale planes in the 14# range. 18 lbs. is too heavy for that engine. You might have to bite the bullet and get a commercially made engine like a G38 or a US41. The Homelite 45 out of their chainsaw would work, but then you have another conversion to do.

I had a 30cc Homelite (almost identical to the 25) in my 17# Tigermoth and it flew it, but it was very sluggish. I put a US35 in it and it made a big difference. I have Homelite 25s and 33s in other planes and they fly very well, but they're around 14 lbs.

Jim