Griffdog1
ArboristSite Operative
PS-520 is a cheaper homeownerish version of the 115i which has a better carb and tune, but a good robust saw all the same. These era Dolmar's are very well built. A good running mid sized Dolmar will go for a very long time without any more than basic care, so it is worth rebuilding if it a saw that you want to keep and use, but you need to work out what caused it to seize in the first place before you start looking for pistons and cylinders. Generally I would say that you are better to hold out for a saw in better condition to start of with, but it could be a good project if you are up for it.