Belize has, without a doubt, the easiest border crossings of any of the Central American republics. No charges on entry, but a US$15 departure tax. Bike entered in passport, plus you get a certificate allowing you to drive in the country, at no charge. There is, however, compulsory third party insurance. But they don´t nap you as they do in Nicaragua and Costa Rica. It's up to you to get it. Cost: US$8 for 24 hours. Being an insurance phobe, I overcame my phobia and bought the minimum, 24 hours worth. At the three police checkpoints I was stopped, their first question was always for the insurance certificate. They seem to be on to guys like me.
If you're in a hurry, you can easily cross the country in six hours, including a pit stop. The main road is excellent. But many speed bumps slow you down. Belmopan is bypassed by a fast road. Belize City can be bypassed via Hattie's Place and Sand Point. Border procedures on both ends about 15 min.