A quick checklist...
Turn rate and radius vs. the enemy a/c-Hopelessly inferior.
Thrust-to-weight/ROC/acceleration vs. the enemy a/c-Hopelessly inferior
Top speed vs. the enemy a/c-better
Under these circumstances, the proper tactic is to use is to use speed to force the engagement when an advantage is held and disengage when at a disadvantage, whenever possible. These circumstances will be true of the P-51 vs. X quite a bit in the MAs. Thus I will not sit here and self-righteously condemn P-51s for running. P-51s rarely run away from D9s and P-47Ns btw.
I WILL self-righteously condemn P-51s for HOing however, since it is a rather fragile airplane with a mediocre armament package that will likely come out on the short end of the stick if the other pilot HOs the SOB right back, instead of trying to avoid go for a more productive tactic, at which point gives the HOtard the ability to say "but it was a front-quarter shot, not a HO".