I only ever used Dolmar's, as such I concur to the already mentioned Dolmar PS-6400 and possibly her stronger sibling the PS-7900....The saw needs to be pro saw between 50cc and maybe 65 cc weight is a non issue reliability is...
I have also read good things regarding the Dolmar PS-6100!
Members usually bash the PS-6400 for being too heavy for its power! It was my first pro saw and I never regretted purchasing her nor did I ever think she is underpowered. Even now having a PS-7900 too I prefer my PS-6400 - she pulls a 20" bar buried in oak with authority.
Also worth mentioning, built 2001., purchased 2006., till now no issues and has never been torn down (knock on wood!).
She will get her cylinder & piston pulled soon for clean up and replacement of the piston ring which got damaged by carbon build up - but currently she is still being used.
My sugestion goes for Dolmar PS-6400 / PS-6100 / PS-7900!