Here is the full list of all 48 MXGP - there are The Official Motocross Videogame achievements.
Select the Pro Physics setting in the Riding Assists section and complete a valid race
Enter Replay mode for the first time
Sign your first contract in the MX1 class in Career mode
Win a race at Teutschenthal in any offline mode
Ride for a total of 300 km (187 miles) in any offline mode
Complete one valid race on each track in the game in any offline mode
Win a race at Matterley Basin in any offline mode
Win a race at Bastogne in any offline mode
Get your picture on the cover of "MXGP Mag"
Win a Championship in any class in Career mode
Win a race at Hyvinkää in any offline mode
Get 10 Holeshots in any offline mode
Win a race at Kegums in any offline mode
Win a race at Uddevalla in any offline mode
Get 40000 fans in Career mode
Win a race at Maggiora in any offline mode
Win a race at Ernée in any offline mode
Get enough Experience Points to reach MX Level 20
Win a race at Beto Carrero in any offline mode
Get 20000 fans in Career mode
Win a race at Valkenswaard in any offline mode
Win a race at Si Racha in any offline mode
Get enough Experience Points to reach MX Level 10
Get pole position and win both races in the same offline Grand Prix
Win a race at Losail in any offline mode
Get pole position in an offline Qualifying session
Earn enough Experience Points to reach MX Level 5
Win a race at Lausitzring in any offline mode
Get the Holeshot in any offline mode
Access your Social Feeds page in Career mode
Achieve a Team Objective in Career mode
Raise your MX Level high enough to unlock a team
Raise your MX Level high enough to unlock a picture
Win a race at Agueda in any offline mode
Win a race starting from the last position
Spend a total of 2 minutes airborne from jumps in any offline mode
Raise your MX Level high enough to unlock a helmet
Enter the Bike Set-up menu in the Pit
Sign your first contract in Career mode
Create your own custom rider
Change your rider's helmet in the Rider Customization section
Get 1000000 fans in Career mode
Compete in valid online races for a total of 60 minutes
Compete in valid online races for a total of 3 hours
Complete an Online Grand Prix
Complete an Online Championship
In Online Season mode, get promoted to the MX1 class
In Online Season mode, get promoted to the MX1 Elite class