You have possibly two options; although I'd hazard a guess only really one.
First and likely your only option; try and get at least 3rd place to get 5 Certificates so that you can use CE or Hex edit the memory address.
Second, and I honestly doubt this will work. After running a race, successfully or not hopefully Sally will offer you her Bandit reward mission. If so, go out and ram your way to a Bandit kill then use the trainer to Max Items to max the skull count. Rinse and repeat as necessary.
Other than that, your only option is to see if another players save game can be used.
If need be, I'll post a speed run (do as little as possible) to a Machine Gun equipped Buggy save game.
Let me know, and good luck.
it really gets the job done, works quite nicely. its not my program but i thank the creator for it.