Aldo, I saw a couple of people reported they got 620 parts from this site:
http://www.rottmansales.com/

The machine is too old to be listed, but apparently they emailed the site with descriptions of the parts they wanted, and were able to get them that way.

There is a guy saying he can sell a download of the Pioneer service manual:
http://store.chainsawr.com/products/pioneer-chainsaw-service-repair-manual

That's about all I can find that might be useful. However if you haven't watched it already, you might take a look at this video: