It’s really quite straight forward. Firstly, find some good legacy FSX, FS2004 or other FS P 51D soundsets.Just Google for them. They are usually all in .wav format. I can, however, point to this one in particular - the startup and shutdown sounds are excellent:
http://www.sim-outhouse.com/sohforums/local_links.php?catid=110&sort=d&page=2&pp=20
Extract all the .wav files to the AH Mustang sound folder together with the Sound.cfg. Delete all non-engine references e.g. flaps, gear, wind etc. Similarly, remove those references in the Sound.cfg file. Then, download MSFSLayout Generator from Github and drag and drop the AH Layout.json folder onto it. The Layout will be automatically updated.
All of the AH sounds will be running in tandem, but they are hardly noticable. You can, if you want, open the AH Sound.xml and change the engine flags from ‘true’ to ‘false’. All the other sounds such as the cockpit canopy, clicks etc. will still be functional anyway.
FYI, I’ve gone a couple of steps further by editing the amplitude of some of the .wav files and cut a bit off the end of shutdown in the suggested sound set. Added complication - that’s up to you. I used WavePad to do that.