In the words of The Penguin Companion to Literature 4: Classical and Byzantine, Oriental and African (1969):
The Thai version of the Rāmāyana exhibits marked differences from the Sanskrit epic. Episodes known from Sinhalese, southern Indian and Bengali versions are included, while Khmer and Javanese versions have also influenced its development in Thailand… Early in the formation of the local tradition ancient Buddhist versions probably played an important part.
The only complete Thai version is the Ramakian of Rama I (1782-1809). Invocations and texts of episodes, some intended for performance as dance-drama, are known from the 18th century. Despite the relatively late date of such manuscripts, the epic has a long history in Thailand. [Edwin Gerow]
At this site you will find “a line by line translation of King Rama I’s version of Hanuman’s journey to Lanka, each paragraph of translation following its Thai text,” and you can hear it read in Thai by clicking on the RealAudio links. An excellent find by Plep.