Converting a Mac powered Gyrocopter to an EA81 or EA82 Subaru requires quite a bit of modifications to the airframe. First of all, the Subaru is heavier, so the rotorhead cheek plates have to be changed out. Second, you will need to increase the rotor diameter at least a couple of feet to keep the disk loading down to a reasonable value.
If you are a light weight person, a direct drive EA81 will probably be adequate. If you weigh over 180 pounds you will need a reduction drive on the EA81 or go to a direct drive EA82 which makes more power.
The best approach to converting the airframe for a reduction drive is to do a drop keel conversion which will keep the thrust line down lower. Rotor Hawk has drop keel conversion kits which raises the pilot and rotor system six inches. That will allow sufficient clearance for 25 or 26 ft. rotors. It will also raise the engine the same amount. Rotor Hawk also has an engine mount. Their email address is: [email protected].
If you want to do everything yourself we have a conversion manual for installing a direct drive EA81 (or EA82) on a Bensen airframe. It includes engine mount drawings. We also have sketches for a drop keel conversion. Cost of the manual is $25.00.