Assuming no other changes from OTL, Sihanouk may or may not remain in power. The coup that deposed him in 1970 was caused by: high unemployment, rampant corruption and anger at their inability to defend their territorial integrity.
Another point to consider is that Cambodia is in a horrible physical position between two rivals (Vietnam and Thailand). So if North Vietnam still annexes the South in 1975, then Cambodia reverts to neutrality or a suitable revolutionary government will be installed at the point of a bayonet point (Cambodia in the OTL in the 80's). Vietnam has an easier path into Cambodia as there are no mountain ranges to impede the advance, Thailand has this problem with their border.
Economically the situation will still be tenuous without a genocide due to economic mismanagement. If this is able to be rectified, then Cambodia could become an Asian tiger. Overall no genocide means that the educated portion of the society are not removed, and this would mean that even if TTL followed a script similar to the OTL without the KR, Cambodia would be in a better position today.
If you would like to see a reasonable POD for this topic, I would humbly suggest that you look at my TL.