I've flown this plane with 2 engines:
the RCF 21cc gasoline w/ Ei -and- an OS91FS.
Use the 91FS, it's a-bit lighter and really lets the plane fly the way the manufacturer intended it to. This is a very light airplane (more in the sport catergory)-[which would make it a great candidate for electric BTW] combined with it's long nose, I'd not recommend the gas engine enless you do the shorter nosed P-36 option.
I've included both pics of My BHM P-36 and BHM P-40. The P-36 using the G20 in her and is a great combo, but heavier than the average bear...FYI-If you're not accustiomed to warbirds-Heavier planes need to be flown more like the real thing (power on all the way to the ground) The P-36 also has Roboart 180 rotating retracts in her. The pic of the P-40 also shows the RFC 21CC engine, but I've since swapped it out...Shown installed are it's spindley wire retracts plugged into the Lado's (which are awesome BTW).
As far as retracts, I suggest throwing the stock retracts away or using them on something else.
I have Lado electrics in mine and am going to be using CJM struts once it gets warmer.
It's a great flying plane and always turns heads.
There's one for sale NIB here on RCU...I's snatch it up for a spare, if I wasn't saving for something else