whats the best engine to put in the hellfire (for racing)
Depending on you skill level, Novarossi or RB .21's like the 21-5 or C5 or 6BB. If your not that skilled the P5 will be more than you'll need.
what is the largest engine that will (for unholy power)
Novarossi and RB again, the 528 and 928.
Having said all that, the stock engine is really good and just past 3/4 a gallon it wakes up. The Axial .28 is on par with the stock engine as well along with a raft of others that are perfectly capable of doing good service in the truck. As said it's subjective in part and depends on how much money you care to spend but think about that because very few will ever be able to get the most out of engines like the 528 and 928 which begs the question, should you spend the money just to have bragging rights or do you want something that will keep you in contention will you concentrate on your driving skilz?