I just wanted to say a quick thanks for the advice people had provided for this project.
The saw is back together and running strongly. In the end, I put in a new piston, bearings, seals and fuel line in from Huztl. The quality of the parts seemed OK. The new piston did seem to be a little heavier built than the original, though I guess that has to be better than too light.
It definately was a learing experience having the saw broken down into every individual piece, and now I'm looking for another one to do it again.
I'm pretty happy with the end result, but I can't say it runs like a brand new one. It does a funny thing where it idles perfectly as long as I'm holding it, but if I put it on the ground, it idles fine for a few seconds, then the revs drop, it starts to vibrate, then the revs pick up again. It keeps cycling like that until I pick it up again. In the two tanks of fuel I put through it, it twice stalled without warning when idling with the bar pointed at the ground, but re-started easily when horizontal.