Hi mitty,
We had one that flew well with a OS 61 pumper, so any good converted trimmer engine should be fine. A Ryobi 31 would be ok, but you may need to shorten the nose to get the CG right. Build it stock, then balance it when it is framed up and you will see if you need to move the firewall backor not. A Homelite 25 would be good and parts are really available.

Dave