On a couple of rare US aircraft engines? I've found the Continental XI-1430 and the Lycoming O-1230, but little more than weight, displacement, and total horsepower. Bore size and piston throw would also be useful (for enlarged versions).
The Continental engine seems to have unrealized potential: it's small, light, and efficient, and for the current period puts out plenty of power. Long term it may not have enough displacement, being smaller than the Merlin, but it has potential.
The Lycoming has the same problem in spades: it's even smaller in displacement, it's flat (not so good for fighters, the Continental originally was a flat engine that was reworked into an inverted V for use in fighters), but it also is small, light and efficient. It was later coupled and used in the H-2470, but only in prototype aircraft.
The monstrous Lycoming R-7755 is also interesting, but mostly in the sense of "Good God, that's HUGE!" I don't know that it's a particularly useful engine, but it's BIG!
The Wright XR-2160 Tornado is also intriguing, but I can't seem to find much data on it.