Fife, I have one with a 160. At first I thought it was marginal on power, But after Jerry Smith and I experimented with props I'm really happy with it now.
I got a heavy one @13lb and with my setup it does pretty good. First off, you need a Jerry smith pipe! That alone gave me 900 rpm. I found the MAS 18-6 woody got the job done best @8900 rpm, a close second was APC 18-6, similar rpm but a touch less pullout, Pro Zinger 18-6 spins up to 9100 but was marginal on pullout. The 160 sips fuel and I'm running 5% so it's pretty cheap to fly.
If you want to go 4 stroke I'm sure the Saito 220 would do a fine job if you can afford to put fuel in it And I'm sure the airframe would handle it.
I thought about putting my rail mount BME 50 on it but I dont think the nose would last too long.
Your really going to like the way it flies. The roll rate isnt anything like a .40 size profile but is very good for an 80 inch bird. Ive never flown an easier plane to hover, I initially had a little wing rock in upright harrier in the wind, but believe it or not it went away when I switched props, The high alpha knife edge is incredible, and it will pinwheel, something I didnt expect from a big plane.
I passed the tranny around in Nashville, and every one really liked it, Gary Seelof said it hung up a little in roll in rolling harriers, and I think Swany said he would like a little more elevator. But everyone that flew it was comfy enough with it to get down on the deck right away.
Heres a pic of Johnny Mojo flying it