Have a car, truck, motorcycle, or bicycle you'd like to sell? List it here for free. Heck, you can even buy and sell tuktuks or your pink Scoopy here. Remember, you need a driver's license in Cambodia for cars, trucks, and any motorbike that's over 125cc. And always wear a helmet!
A Suzuki DRZ400 will be quite light and good for long distances, although won't have the same ability to carry a passenger/lack of panniers etc.
Quite a few Versys around and I think that would be the best choice (unless you really want 21" front wheel). Same for Transalp 400-650 and Africa Twins (though those aren't very light). Many available. You might also consider one of the smaller BMWs (650-800cc).
As mentioned the CRF Rally is a good option, although a little under powered at times. Keep in mind you're not riding as fast here as in Thailand, so there's never a big need for significant power.