While you're on 'Main Menu' press:
R1, R2, L1, L2,  R1, R2, L1, L2.

A keyboard will appear.

Type: WORLD CHAMP

This unlocks every feature in the game.
All bikes, tracks and challenges.

If you go to Lightweight/ultra lightweight TT, you'll find the 400's.

Set the handling to 'Expert'.
On Intermediate it does a lot of the braking for you, and in my opinion can be a bit confusing.

I find it easiest to use the onboard view, so that you're looking through the screen.
While actually playing, when riding push the analogue stick forwards to tuck in.
Then roll it around to steer.

Hope this helps a bit