How about having a rubric to determine a point value given to certain aircraft based on several aspects of it such as speed, turning, climb rate, acceleration, and firepower.
This would be complicated to produce, but it would put an accurate point value on a plane better than ENY, I.E. Spit5 would have a higher point value than a P51 because it can turn better and has better guns
Here is an example:
Speed @ 2,000ft
-300 1 pt.
300-325 2 pts.
326-350 3 pts.
351-375 4 pts.
375-400 5 pts.
400+ 6 pts.
These values are skewed, but hopefully you can get a general gist of what I am saying.