I'm converting same kit to electric with 16 Sanyo 1950AUPS and geared Astro 25. I have flown three versions of the P51 kit with glow 2S 40's. Speed was around 60mph full throttle in level flight. Needed gear bent forward so axles were even with leading edge to avoid nose-overs.
Needed some down trim for level flight at full throttle, no down trim for landings. I imagine that a 4S 46-52 would be ok. I don't like trouble of inverted engines , exposed ugly cylinder makes for better cooling, easy starting and easy maintenance. This plane can make you look good!