for now it's still possible to actually use the old models.
Although there's a few glitches (brake lights not working etc) you can drive your stadiumcar in canyon.
how? really simple actually.
Do the same as you're used to from tmnf/tmuf.
Download a model (not sure if .loc still works, but to be safe download that one too)
put the .zip and the .loc in the My Documents - ManiaPlanet - Skins - Vehicles - CanyonCar
now launch the game
go to Editors - Manage Packs - (on bottomright click ADD)
now go to Skins - Vehicles - CanyonCar and select your car
press Create and Voila, game asks if you want to use that model ingame
Now you're done and it shows ingame