It's pretty easy to ride through Angkor, just purchase a pass for entry and if they don't let you in the front gate, ride around to the other side of the park. The guards don't usually care about the motorcycle, they are more interested in collecting the rather steep entrance fee. Haven't been since 2008, but it was something like $40.00 USD for the 3 day pass. Not sure how much longer they will let people roam freely, as the temples are really taking a beating from the tourists climbing all over them.

Cheers!
ChinaV