Look for a Sachs based chainsaw, or one of the larger Sthil chainsaws. Anything over 50cc is hard to find, and usually pricey as you are getting into the "profession grade" engines.
Poulan 3300 or 3500 are also worthy. The Echo CS510 is the basis for the BME 50. I have a converted Poulan 46cc engine that I'm very happy with. There is a lot of info in this forum, just do your research.
Z