Try to talk to somebody at Hobbico first. You need to expalin to them that you think you need to fly with a larger engine due to the density altitude you'll be flying at - let them know about that 9000'. Last thing you want is to find out your plane will fly at 60 mph ground speed, and just mush along.

I haven't read the assembly manual, but per the Tower web page, it should work if you something like a .40 LA. They warn against the FX engine which has ball bearings and will weight a lot more and power.

Hogflyer