about 1220 The Khmer lost important parts of Thailand in a rebellion and were eventually defeated by a later and more powerful Thai kingdom, if they hadn't lost the Western provinces it is doubtful they would have been beaten by Thai attacks.
The Mongols were avoided by large annual payments and wouldn't pose a problem.
One of the main problems for the Khmer empire was that they were incredibly reliant on their system of canals and reservoirs and when they flooded the empire was substantially weakened If they didn't happen in TTL The empire may have survived.
From this position it would simply have to exploit Chinese weakness after the fall of the Yuan (Mongol) dynasty and claim arable land in the north that could alleviate its problems with agriculture. If it retained those conquest it could have bought the technology needed to modernise from the Europeans, as there were several large urban areas already they could have modernised quickly and possibly make an alliance with Japan and carve up China between them.
Maybe...