The CG is at the forward wing spar. I had a few hundred flights on these things with a Rossi .81 and an O.S. .91. Does your kit have the elevator servos mounted in the back next to the stabs? If so, then the O.S. .91 will work fine. If you have the old kit without the rear formers that the stab servos are mounted to (older version) then your vertical stab will flutter at full speed. You need to have a bulkhead that wraps around the thrust tube and goes into the vertical stab....otherwise it's flimsy as hell.
Good luck,
Dave