I have a 510sp. Very similiar saw. I like it allot. Very light, especially when you consider how old it is.
Parts can be difficult to find. Mine is getting tired (compression around 120)
While I was able to locate a new piston ring (they only use one), I can't find a gasket and piston pin clips. The oiler works well, but if it fails, the cylinder needs to be removed for servicing it.
BTW, I run a 18 inch 325 .050 on it and it likes it. Check compression before you buy. I paid $75 for mine last year and it came with 2 bars and chains (18 and a 20). Even with 120 compression, it cuts fairly well, it would be a great saw if I can eventually get all the parts for a top end redo. KD
If you do purchase it, send me a PM and I'll make a copy of the factory service manual, it covers the 510/520sp