Looks like a typical late model Sure Start product motor built in the mid-1990's. That exact engine was in some of the last PT-19 Trainers that were built. Also in the Extra 300, except that one did not have the exhaust shield installed. And probably in a lot of others. In any event, it is a good general purpose 1/2A engine with decent power from dual ports. The weak spot in these would appear to be in the journal bearing, where the crank may spall the aluminum bore. Good fuel is a must. Polishing the crank can help if the surface finish is not as good as it could have been. The later model engines were not manufactured to the higher standards of the earlier engines, which in my opinion were simply jewels. Talk about precision....
One thing to be aware of is that these engines may have gone thru several owners and been reconfigured in non-standard ways. I do it all the time to use the best parts or get a configuration I want. Even the factory made all kinds of weird configurations at times to use up parts? (For example, the brass drive washer was for pusher applications, but seemed to start showing up at random on other applications.) My next engine is for a P-40 Guillows model, and will have a product engine case, crank, drive washer and ratchet starter; a Killer Bee plastic backplate but the needle from a sure start since I need the extension; a ground-OD cylinder with side port muffler to get the exhaust out of the cowl, and a standard glowhead. Hope I can get back to work on it sometime soon....
I don't think it was a matter of degraded precision. You probably already know this, a historical summary is contained in
https://en.wikipedia.org/wiki/Cox_model_engineI am told that when the Estes Corporation, the model rocket company purchased Cox Engines in 1996, from that point they were basically assembling engine from left over parts. Cox had a few key ladies who knew how to quickly match cylinders to pistons with the best fit, Estes did not. Thus, there were engines that had poor compression as a result. Plus, they mixed and matched parts where that if one got a so called "Venom", one of the hottest Cox reedies could have received something less than that. As they ran low on parts they substituted. So, you may find a conglomeration there if from those Estes production runs.
Later engines are being cobbled together by Cox International Canada and Ex Model Engines along with a few others. However, they seem to be doing a better job in their assembling, plus doing limited production runs through subcontractors where parts are short and there is still a market for them.
Their costs may seem high, but we no longer have the mass produced quantities that Cox of California was pumping out in its hay day, which kept costs down. I've used an inflation calculator to compare costs. Their costs are a touch higher but not by much. (We also had more disposable income 40 years ago as taxes were lower then.) They are also disadvantaged by local labor costs, which are by nature higher than costs of labor overseas. It's all a part of local cottage industry environment that CL is now surviving on (IYKWIM).
Estes was on the way out in 2008, April that year I bought 2 Sure Start engines from their surplus store for $6.99 + $7.99 shipping. Then their engine shop closed completely in 2009. Various buyers to include Cox International and Ex Model Engines along with a few others purchased their remain inventory to include engine parts after that.
But you are right, when buying things used, no telling what a previous owner may have done with an engine. Because parts are interchangeable, I even remember back in the 1970's buying parts from Sears, Montgomery Ward, etc. If they were out of stock with one cylinder piston set, I'd just buy what they had on the shelf. Then, Cox racing fuel was readily available in the stores. if the engine wasn't running with enough power, I use that. So mixing and matching becomes possible. Taking a donor engine, removing the plastic backplate and replace the worn tank engine got someone back to flying again.
So, it is possible to get a surprise or two when buying stuff used.