If I were you a solution to your problem could be answered by the individuals who have powered this airframe with a .90 size powerplant. I believe there is another thrend on this airplane were they have used this size powerplant.
The supplied engine mount will accomodate up to a .60 size engine but is not suited for a .90.
You will have to purchase an aftermarket engine mount, redrill the mounting holes and reinforce the firewall. I have found through experimentation that my .60 performs best with a 14X4 wood prop. However, I still don't have enough power to pull out of a hover.