I'm using a laser 80 in mine, its is still on the building board. The engine fits entirely within the cowl inverted so only cooling required.

[link=http://www.laserengines.co.uk/usa.htm]www.laserengines.co.uk[/link]

It may seem a bit expensive but you don't need any additional in cowl muffler. It has also allowed my to move my throttle servo and battery pack in front of the firewall which should help with the tail heavy nature.