Assign every plane a perk value. The value of a kill is determined by pro-rating against your plane as below:
KillValue = Min (TargetPlaneValue * (TargetPlaneValue/YourPlaneValue), MaxKillValue)
This is to reflect that it's easier to kill an early-war plane with a late-war plane than vice versa, but we give a maximum score to prevent anomalies.
Anyone can fly any plane, but you only score for the excess over the value of your plane. So let's say you take up a Me262 with a value of 100. You would have to kill 101 pts to score 1 pt. ie
FinalScore = TotalKillScore - YourPlaneValue
OK, it's a bit rough, I know.