$50.00 is a good price for a running 630!! As Lee said, Hope yours doesn't go in for repairs to the tune of $200.00!! However the nice thing about doing repairs is whatever you repaired should be as good as new and you shouldn't have to worry about that part for the realistic life of the saw.
I've used Lee's 630 and it is a real nice saw. The only other 630 I 've used is my own 630 Super II which I got for free!! However it was NOT a running saw at that time. I had to go right through it. New trigger (E-Bay) and used handle trim pieces (Thanks taplinhill!!) all new AV mounts (Hard ones from Bailey's) new Meteor piston (also from Bailey's) used 670 airfilter cover (E-Bay) rebuilt the carb, new throttle linkage (Thanks spike!!) and many hrs.
I did a bit of porting work to the piston, exhaust port and smoothed and matched the intake parts. Set the squish to 0.022". It is broken in now and really rips. It has an 18" bar with a near new Sthil chain. It has a lot of grunt but also turns up very nicely. Runs best at about 14,500 WOT no load and 4 stroking to beat the band. At first I tried to slow it down a bit by richening the H but at lower rpm (spec 13,000) it cut ok but it burnt a LOT of fuel. So I turned it back up tuning by ear and it's right back to where I had it in the first place. It runs super and gets much better fuel economy. For now it has become my "go to" saw. Good luck with yours!!!