Lanier Predator
I recommended the .40 because that is a common engine that is being raced with around the country.
Not sure if you ever intend to race the plane, but if you went with the .40 you would probably be ok.
Locally (In Minnesota) we are one of the odd ball groups that use the O.S. .46fx which is also a good engine for the plane.
If you have no intentions on racing, just about any of the sport .40 or .46 engines would be fine. In fact, we use to have a group that flew them with .25s and they would still go 90 mph.