I'd check out Werks and Alpha engines. Incredible engines for a little price.
Do not let the price fool you
As for alpha, the red & green head are perfect for buggies. On the other hand, the orange one is best for truggy, as it's got more torque.
The Werks engines, either a B5, B6 of B7. The B6 is an optimized B5 with a smoother power band and more fuel mileage.
You could also check out the topics on this forum.