Entering Cambodia from Thailand should probably be done at the Chong Chom/O’Smach border in Thailands Surin province. (and then theres only 2-3 hours ride down to Siem Reap) Sa Ngam a bit east of Chong Chom is also ok from what I have heard. I had a friend crossing over there with a thai plated bike earlier this year without problems. Maybe Hat Lek will be ok too, but Im not sure. Others might now better?
Do not cross at the Poi Pet border, its corrupted as f*%# and they would want expensive permits.
Crossing into Laos on a 125 cc bike is since a year or two back not allowed. It must be at least 250 cc. But how strict it is enforced at each and every bordercrossing is hard to say.
Thai bikes are also for the moment not popular into Laos. Must be a part of a group and have permits via a Lao agency. David would know more about this...