After having a good chat with the same mechanic (who seemed to think he never said "they just get tired and need a rest") and taking the head off and shining a torch down the barrel it appears there are a few scores down the cylinder wall and these partial seizures are definitely happening. I can definitely feel the scores with my fingernail. The spark plug is showing no signs of a lean situation, the carby is good, oil pump is good, thus i think it's just too worn out in there, evidenced by the 3 to 4 scores. So I'm going to attempt a top-end rebuild and from what I understand it looks pretty simple.
I've measured the cylinder and it seems it is at the original 49.5mm. Which means in 18,000kms and in 33 years it has never been re-bored, not sure what honing will do to cylinder size (?) so I'm not sure if it has had any attention to the top end at all.
Now I have to find out whether a hone will do the trick with new rings and gaskets or if I need to go the re-bore and buy an oversized piston and rings.
Any advice for a noob? I am yet to take the piston off and I have the hole covered with cloth but I'm keen to get the piston off to take a look / play around. I can't seem to find a 'piston pin puller' in any stores around here and one guy said a hammer/mallet and a bit of care will do the trick. Can anyone give me some pointers here?
I'm quite enjoying getting some experience and I'm looking forward to attempting a tour up to the Flinders once this little thing is up to scratch