The Petter PC range was never that popular (particularly in multi cylinder format), as is indicated by the difficulty in obtaining parts. Though there have being a few parts surface on Ebay UK lately.
These engines are very similar in appearance to the Petter PAZ1 engine, and do infact share a few common components (valve guides, and maybe piston rings and valves from memory)
I have an electric start Petter PC2 for parts reclamation if your interested, though the part you are seeking is not present, because it was electric start
.
Send me a PM if this sounds interesting.
The only other option would be for you to try :
http://www.petterengines.co.uk/Also see this thread on a UK forum:
http://www.stationary-engine.net/forum/showthread.php?t=3701