This is the strongest 18 size engine out there that is still in production that I know of.
Here is the engine and parts needed for it.
http://www3.towerhobbies.com/cgi-bin...?&I=LXHLV9&P=0 engine
http://www3.towerhobbies.com/cgi-bin...?&I=LXJPZ5&P=Z rear cover. you must cut the pin off the crank for the pull starter.
You will have to do some mods on the carb.
1. cut and reshape the intake so it clears props
2. turn carb around so that the needle points at the carb
A. now take apart the needle and remove from the carb
B. cut the top part of the brass on the machined nut section. cut through the middle of this. then solder a small brass circle on top and when cool use as a nut to hold the nipple on.
C. reattach to the carb
3. Use this remote needle as its small and easy to use
http://www3.towerhobbies.com/cgi-bin...&I=LXDG58&P=ML
4. You will have to make a special nut that goes down into the prop. this centers the prop and hold it on.
This engine set up with a 21cc marine pipe will turn a APC 6.5x6 Pylon at 26,500-27,000. enough to blow the tips off. I Havent flow it yet as I need some carbon props in this pitch range and a plane to handle it lol.