The .55-AX is one of the best running, most reliable engines on the market. I have ran them in cowled applications with zero issues. The key is airflow. air has to be directed over the cylinder and the air exit should be at least 3x the size as the inlet. A lot of people make the mistake of an exit that is too small, myself included.

It definitely sounds like there is something wrong with yours though. Hopefully you'll be able to figure it out, they are great engines.

As for the .75-AX I imagine you can make it work. People overpower airplanes all the time. I personally wouldn't do it but that doesn't mean it won't work. Just be prepared for CG issues and heavier wing loading.<br type="_moz" />