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
Win a race with manual transmision
Defeat Dennis “Salesman” Rockmond and take away his girlfriend
Defeat Billy Bob Sykes and take away his girlfriend
Defeat Ricardo “Azteca”Vargas and take away his girlfriend
Defeat Sergeant Alec McAndrew and take away his girl
Win a Escape event
Win a Pursuit event
Win a Stunt event
Reach rank 10
Get all visual improvements for one car
Reach full car performance improvements
Win a Knockout event
Defeat Brian Wetherby III and take away his girlfriend
Buy your first car
Win your first event
Win a Race event
Win a Sprint event
Win a Time-attack event
Win a Lap record event
Win your first gold medal