There is downthrust built into the firewall installation. I don't remember the exact figure but it is somewhere around 2 degrees. I have a Saito 91 in my Hog and it flys fine. The elevator is centered when trimmed. Try putting a couple of washers under the motor mount to give some down thrust. Check your wing incidences as well. Both wings and the stab are set at zero degrees incidence, while the engine downthrust, as mentioned, is around 2 degrees.

Cheers!

Jollidude