In Guatemala City there is a BMW dealer called Bavaria Motors.
I have seen it on or near the Reforma, but do not have the exact address. But once in the city, try any of the many bike/car dealers and I am sure that someone will point you there.
Drive into Guatemala City down CA1. After a couple of miles it becomes Boulevard Liberacion. On your left there is a Volvo/Kawqaski dealer. Couple huindred more metres, follow signs taking you left to Reforma. You're almost there.
Hear good things about them. Retail price for new bikes is, apparently, competitive with US.