I have read about gas engines used in Pattern and I think there is a few issues at the moment . .

1/ current 30cc gas engines have nowhere near enough power for competitive Pattern
2/ a lack of available headers (and perhaps pipes?)
3/ a lack of available soft mounts (which would be desirable in a composite airframe)
4/ a lack of test data and set-ups to copy

Most of the gas engines I have heard about are the ZDZ40F3A . . and I think physical size and weight is an issue with them. . . oh . . and I believe the vibration on the airframe is a little savage?? . .

Cheers, JB