The King Air is indeed in all three "main" sims. FSX, X-Plane and MSFS2020. With P3D being based on FSX I'm going to assume it is still in there. It has been a while since I've run P3D. I haven't really fired up XP12 so I can't answer there.
With that said, I do agree that creating the event with a selected aircraft like the King Air and taking off the airspeed restrictions would "even" the playing field a bit. As well, having the same aircraft in the sims discounts for MOST differences in folks' setups. My concern was someone flying one aircraft on a high end system cranking out 80FPS would potentially "beat" somone with an older system just maintaining 20FPS and stuggling to manage the sim, addons and other apps. Just my take on that.
I would also add a restriction that AP shouldn't be used. If you are cruising through a mountain pass you REALLY should be awake at the controls. I believe this would also disaude andy one from taking off, popping on the AP, then wandering off somewhere else or goofing off on facebook while the sim runs in the background.
And I agree that AAS should be clicked on right before you advance your throttles for the take-off roll. Having to grab the mouse and go click the start button/menu on AAS while doing your climb-out is asking for trouble.
Though clicking it right before you go full throttle AND the fact that the departure point is the same, the distance to the end of the runway will all be equal. And if you add in that each "ticket" would be flown with a specific aircraft again levels the field. And of course AAS should be turned off at the point you reach 0 on the runway, click stop, then move on to taxi off. this would take into account folks who land with "flare" versus a "plop and drop". And for the folks who think they are doing carrier quals landing at full throttle and burning out the brake pads before they run out of runway.
With the aircraft restriction I would create a few tickets with a couple of different aircraft. One series for say the King-Air, another for a C172 and maybe another for a Baron or Mooney. Get a consensus of what DEFAULT airfraft are avaialbe in the sims and what folks are apt to fly. Then of course you COULD create another ticket for one of the more popular payware add-ons.
I know that seems a lot of work to put on the shoulders of the Air Managers, but being a race you really don't want one person cruising along in a Cessna to be consistantly beat out by the one yutz that is flying the Darkstar (Mach10 jet from Top Gun 2 and is in MSFS2020). I know, that is what the airspeed restriction was for, but like Simon mentioned that sort of beats the purpose of it being a "race"
If you get two or more people flying the same bird, but one person is more adept at managing their engines, rpms, and all the other bits and bobs of flight might just come out on top becasue they efficiently flew the course and beat out the "speed demon" who had to cut throttles or had issues maintaining airspeed becasue they were over working their engines or props and just flew like a brick in the air. Again, just my take on it.