I recently completed a Phaeton 90 for a hobby shop that included a G-26 for the power plant. It required few modifications for the engine installation. You can forget about installing the cowl, though. It flys very well, with an abundance of power.

The biggest problem was avoiding cutting down the engine back plate that comes with the engine. Since it was not my plane or engine, I chose to make a slightly wider and taller bolt on firewall that attached to the existing firewall to accomodate the G-26. That way the owner could change to a different engine if desired and still maintain the stock, unmodified, G-26 back plate for another plane.

You may want to re-locate the flight servos to the tail for better balance purposes. I did, and it made it a lot easier to get the correct C/G with only the throttle servo and flight battery in the fuselage at the wing saddle. Just add some 3/32" lite ply panels at the aft section of the fuselage for support around the servo openings.