Thanks AVB and Norm,
I got an aftermarket piston and rings for it for $52.00 here
https://www.ebay.com.au/itm/234632240327. The old piston had 1 ring new one has 2, I don't know if that would make any difference, but the saw seems to run quite well and has plenty of power. It was surprisingly easy to get the pin bearings in the first time but as I only seem to learn by doing things wrong, I put the piston in backwards and dropped a couple of pin bearings getting it out and back in. A little harder getting the pin bearings in with the piston in the way and the grease I used wearing a little thin but kept me entertained for a while, could have used the special clamp mentioned in the instructions which AVB supplied.
Another tip for amateurs, if your fixing up an old saw that has had a little abuse and not been run for a few years it would be a good idea to clean the fuel tank before starting it. In this case it ran well until I got to the tree then it quit, and would not restart, a little embarrassing as I had an audience. The fuel filter was completely blocked and allowed some stuff to get through to the carburetor. Fortunately it responded to a bit of carby cleaner in the jets and I was able to prune the tree, after the audience had gone home.
Again thanks to AVB and Norm for your assistance