Maybe look at it this way; if you buy an old UK CS engine, irrespective of where you buy it, view it the same as a new Listeroid from India. Work on the idea that it will need a complete rebuild.
Instead of sand in the crankcase you’ll find years worth of thick black oil that has set like tar. This stuff is as black as black gets and transports itself everywhere. Invest in some good ear protectors; you’ll need them when your partner discovers it on the best house carpet!
You won’t find any badly made parts or ill fitting castings (if all are original) but you will find parts that are worn out and castings that have cracked.
CS engines will still start and run with incredible amounts of wear on the bearings and rings but if left too long it ruins more expensive parts. Budget for at least new rings, bearings and a set of gaskets but don’t be too upset if it needs a re-bore, new piston and machine work on the crank.
It’s worth remembering that most of these engines are now over forty years old and, in their later lives, have often been ‘serviced’ by folks who are trying to get the last bit out of them, just like we’ve all done with old cars. I’ve seen all sorts added; piston rings from gods knows where, bottom big end bearings drilled to replace a top, rag oil seals in cracked main bearing casting ends, wooden shims in worn cam bearings and many castings that have been fashioned from parts of old tractors. I could go on, but my glass is empty!
If you buy an original SOM unit then anticipate a re-wire as minimum.
On the bright side, once you’ve rebuilt your engine, it will last a long time. It may even last longer than me, and I’m 94 this birthday.