RE: Phoenix Dolphin 40
I have built and flown the Phoenix Models Dolphin .40 ARF. I put a Magnum XLS .46A 2-stroke on mine, and it flew wonderfully. The hardware and covering are really terrific. My only issue was with the soft, bendable main landing gear.
I won a pattern competition with mine (sportsman level, of course), and even did some Club 40-style pylon racing. It's a terrific airframe that flies as good as it looks, and it's an outstanding value for the price.
I have been happy with all of the Phoenix Models ARFs that I've flown over the years. I've had the Dolphin 40, two of the SeaBee ARFs (one of RC flying's best kept secrets, really a terrific airplane), the Sonic High Wing, the Sonic Low Wing, and I have a Giles 202 up next on my assembly list.
I would expect you'd have no problem installing an Enya .53 4C to the stock motor mount.
I've seen the Texas RC Planes Lone Star 50 ARF; it would be a nice choice, too. It's pretty much an homage of the Tiger 2, however, and might prove to be a little trickier to mount a four cycle engine on it if the motor mount set up is hardwood rails like the Goldberg it is derived from. Hopefully gmeyer-RCU can clarify that point; I haven't seen the motor mount setup on the Lone Star 50 up close.
Good luck and good shopping!