The JK6 engine was never produced in vast numbers, so the more unusual spares will always be tricky to locate, the best chance of finding these parts in the UK would be Sleeman and Hawken.
Is it anything like a JP4 governor?
From speaking with a retired Lister Petter service engineer (who at one time worked on the JK6 assembly line). He is of the opinion that there is no commonality between the JK6 and JP4 engines in this respect.
From the same source, I may be able to get access to a JK6 workshop manual (if he can find it). Hence if the owner is able to expand on what they need, I could at least quote some part numbers for reference purposes.