OS46AX with a 11x5 up to a 11x7 prop. Increase the throws and seal all the control surfaces. I also eventually converted mine to a taildragger but made it so that it could be used as either setup depending on the need. Plane still flies fantastic even though I no longer own it. (I am training the person who bought it from me).

Replace the stock main and nose gear. I used Dubro super strength main gear

http://www.shopatron.com/product/pro...d=DUB789/101.0

and Fult's twin strut nose gear.

http://www2.towerhobbies.com/cgi-bin...&I=LXH124&P=ML

and ended up with tons on ground clearence. With the Fults nose gear I listed you can use the stock firewall mounts or their own if you haven't gotten that far in the kit yet.

At one point I put on an APC 12.25x3.75 prop and the plane went nuts. Lots of vertical (not unlimited) and all other maneuvers got more responsive, but throttle management was a must since this prop creates tons of pull and full throttle takeoffs were rather spirited.

This picture only shows the main gear.