I have here, a "big bore" 56mm Chinese cylinder saw - all Stihl bottom end, pretty much the rest of the saw has been made with parts taken off saws bought to me for repair- where cheap parts were swapped out for old secondhand OEM ones- the collected take off parts put together as a saw, wearing a new big bore cylinder someone wanted fitted to their saw until we did so and ran it against some OEM 54mm saws- it did not produce the power to keep up let alone beat the OEM saws.
So, yes, you can buy cheap and make a saw run- it just might not run as well as a good 064 or even 046/460 can and certainly will not last as many run hours or seasons.
New OEM, good 2nd hand OEM, Meteor, Hyway are my selections in order of preference for quality and power produced in standard bore sizes with no tinkering done to port timing or machine work- just bolt on and play.
Advertise on here- someone will have a secondhand top end for an 066 or 660 for sale.
In my opinion- if you feel you need "better" than 660 spec for some reason- try to find an early non decomp 066 cylinder to put on your bottom end, they were perhaps the best power wise- but no decomp can make for interesting cold starting.
Like Gord says, check out the bottom end. Not sure what it is like where you live- but here most old missing bits 066/660 saws are ex commercial forestry saws and have done huge hours before being retired- huge hours = wear. You need to confirm all bearings and oil seals are good, case is not suffering damage or stripped out holes for all the bits that bolt on, cracked or wallowed out AV pockets and the like.
066/660 does not need big bore upgrades- they are a damn fine saw in original spec.