Here is the full list of all 21 Motorama achievements. It takes around 3-4 hours to unlock all of the achievements on Windows.
Buy one car for each class
Defeat Sergeant Alec McAndrew and take away his girl
Win a race with manual transmision
Reach rank 10
Defeat Ricardo “Azteca”Vargas and take away his girlfriend
Get all visual improvements for one car
Win a Stunt event
Defeat Billy Bob Sykes and take away his girlfriend
Win a Escape event
Win a Pursuit event
Defeat Brian Wetherby III and take away his girlfriend
Defeat Dennis “Salesman” Rockmond and take away his girlfriend
Reach full car performance improvements
Win a Knockout event
Win a Lap record event
Win a Race event
Win a Sprint event
Win a Time-attack event
Win your first gold medal
Buy your first car
Win your first event