See below, if you've gotten that far, you should just need to hit play one more time from the main menu, and the ending/credits will be triggered.
If your experience doesn't match up, let me know and I will see if I can find a bug around where you are.
ENDGAME SPOILERS!
After the "new" Frizzle shows up, there should be some dialogue between Frizzle and the narrator, after which the narrator will tell you to crank and delete Frizzle. You can either crank, or not, and the game will "crash" and send you back to the title screen.
After this, hit play one more time to trigger an ending based on whether you deleted Frizzle or not, and the credits will roll shortly after.