Had this argument with ex-mother-in-law many times; I could clearly hear Western 50s/60s songs in Sin Sisamouth (& others) songs of the 60s/70s, but of course he did them first bong!

Also, the Beegees formed in 1958, Massachusetts was released in 1967 - however, the Beegees wrote the song originally for a band called The Seekers (Australian), and that band was around 1962-67, with a peak around 1964, so perhaps Massachusetts had been heard in '64 somewhere/somehow before 1967. I don't know the date of the Sin Sisamouth song, but I'd wager it was after 1964, and probably even after 1967.

The US influence came into Cambodian music after the US put ground forces into Vietnam in 1965 and set up radio stations that could be picked up in Cambodia.

Here's another example, Sunny by Bobby Hebb, first recorded by a Japanese artist in 1965 and a hit for Bobby the following year. The lyrics are usually completely changed in these songs to fit Khmer language timing.