I would say there are three viable options, buy a brand new engine, or a just zeroed one from a trustworthy source, or get an engine at lifetime end and let it rebuilt at your preferred engine overhauler. I personally would not go for a used used engine, because you have no idea wether it was broken in correctly and/or how it was treated by the former owner. This is a bit "certified thinking", but I firmly believe it is best also for experimental and homebuilt, at this stage.