ORIGINAL: checkmate91
Here's my perspective: my big heavy trainer (which is quite acrobatic incidentally) was fitted with an OS40 LA and variously 11x4 or 11x5 prop. fine in air, rubbish on ground when grass anything other than shaved. I fitted an ASP 52FS (magnum or SC in the US) with a 11x6 prop and the thing just leaps off the ground. it's great. Now, the os40 produces 1.0 hp the 52FS produces 0.9 but the torque is completely different. Don't get me wrong, I love the simplicity of my little OS 40 but love the sound and the grunt of my 52. I think some of the other correspondents have suggested a 70fs (or 80) in your p51, that would sound great and if your kit asks for a 46 it would do the job...
It's not nessesarily the "torque" that's responsible for this. That .52 likely just makes more horsepower than that LA 40, period! The real question that needs to be asked is "how much horsepower does a LA 40 make at 11,000 rpm, with a muffler?" My best guess is no more than 0.7. When O.S. says that the LA 40 makes 1 horsepower, I suspect that they mean that a test engine managed to make this much horsepower on a really cold and dry day with an open exhaust and a tiny prop that let it scream to 17,000 rpm and it managed to do this exactly once in a row. Or, someone could have just pulled that number out of his rectal cavity.
The fact that 4-stroke engines make their peak horsepower at actual usable propeller rpms means that when they say ".9 horsepower", it's .9 horsepower that a sport plane can actually use, not a hypothetical horsepower that is only available if you had some kind of reduction gear on the engine to match the engine's ultra high rpm to a efficient prop turning at 9000 rpm.