Series 3 engine (engine numbers 9??*****) - why not just buy a Series 2/2A rocker cover
Other than the plastic orange oil filler cap the engines are pretty much the same.
Over 50/60 years the engines wear or breakdown and many owners will have "upgraded" to a newer Series 3 version.
No big deal unless you want a rivet counting as-it-left-the-factory-special.
Buying an engine off eBay is very much buying a pig-in-a-poke.
You have no idea or any guarantee of what you are buying.
You can not trust what the seller says about it.
Much better if you can see/hear/drive the engine in question, before removing it from the vehicle.
If the aim here is to buy a decent engine as a long term investment/keeper, then buy cheap and get it off to one of the recognised engine re-builders and pay £2500 ish to have it made good.
If you buy an engine from between £50 to £2500 you may still have to spend £2500 to make it good.