You could build a plywood box to space the motor the correct distance from the fire wall, a lot of the 3d planes use this method. About 15 years ago I built a plane that used 2 ply boxes, one fit inside the other. You bolt the motor to the large box and slide it over the smaller box to get the spinner space you need then bolt or glue them together. The boxes were 1/8 air ply and the motor was a ZDZ 80. It worked fine and had no standoffs. The carb was inside the box and got air from inside the fuse. it had a hole on top for needle valve adjustment The box was also used to mount the ignition box and the servo and micro switch for eng. cutoff. I glued furniture grade hardwood corner stock inside all the corners for strength and aluminum angle to reinforce the motor mount ply wood (aircraft grade) And the box to firewall joint-It isn't necessary if you don't use a ZDZ motor but may be if it is a bigger brushless motor.