If it wasn’t for Packard, the 51 would never have been the successful plane it was. I got a chance to visit with a client who collects packards and he has a still airworthy Merlin on an engine stand in his collection. What an amazing engine! The Packard built merlins had a better time to overhaul than the UK produced motors. I’ve been poking the owner to try and get him to bring the motor up to functioning and put it on a stand for demos. The fact I got to touch that engine was a real treat. I really want to hear it run. The previous owner almost put it in a miss Budweiser boat where it would have eventually blown up. He couldn’t bring himself to grenade a still airworthy Merlin.
I think even if the Mustang had kept the original Allison V-V-1710 engine, it would have gotten faster anyway because I remember reading Allison had plans for major improvements in forced induction to correct the problem with higher altitude performance. Probably by early 1944, the V-1710 would have a two-stage supercharger, which probably would have made the P-51 only slightly slower than the V-1650 powered P-51B at altitude.
We actually have a couple that fly into and out of our small airport. They flew on Sundays(my golf day) and it was pretty cool to see them flying around.
It is my understanding that most of the owners of Mustangs have at least 1 spare engine available. Or 2 or 3. It’s a rich mans game.