So i figured out you can copy over your CarMODgeddon folder you have already setup and used in Reincarnation (meaning you used the Minge tool and activated all the cars you want and such) to Max Damage and they will work fine. There seems to be an issue though, you will notice a lot of cars engine sounds don't work. well i have figured that out too!
Stainless has changed the names of some engine sound files in:
C:\Program Files (x86)\Steam\SteamApps\common\Carmageddon Max Damage\ZAD\audio_engines.zad
you can open this "zad" file with 7zip or an equivalent program once inside the archive go into "audio\fmod" you will see these files
now that you have that open you'll want to navigate to to archive of the modded car you want to fix the engine sound of in your "carMODgeddon" folder you copied over from Reincarnation and also open that in 7zip. Look for a file called "vehicle_setup.cfg" and drag it out of the archive, this is a text file and you'll need to edit one thing in it and then put it back in the archive.
Now open "vehicle_setup.cfg" in notepad or even better Notepad++ (its what I'm using here) and find the line below.
it won't say "Euro_V8" like mine cause i already changed it.... but it'll say something like "Camaro, Maserati, Jeep, Ferarri, ect....)
you'll need to change these to a name that matches a name of a file in the first image. you'll have to use some common-sense as to what engine sound to use, like if its a big truck use "SUV" or if its a car "Euro_V8" or "sportscar" is a safe bet, any of them will work.
hope this helped until the modding community updates their mods and tools and makes it easier.