Hello. 'fraid I can't remember costs etc, to enter Belize from Guatemala, but it was very straightforward. And all English speakers.
If you're going round central america, you'll have a much easier time on a bike with local plates. You pay nothing at the borders, and go straight through. Compare 4 hours to get into el salvador on a US reg. bike to 15 minutes with Guatemalan plates.
Small enduro bikes < 350cc are easy to find pretty much everywhere. For something larger, try the capital cities.