RE: Bought a used 40 sized P-51 has issues.....
A 6lb plane with a .46 LA engine is going to be relatively UNDERPOWERED.
The LA may not have enough OMPH to get the plane through the loops.
A .46 LA is about equal to a .32 GMS or FX engine in power, which I would use on a 3lb sport plane at most.
It's fine for a .40-.50 trainer though, which is not expected to do things like loops from level flight.