Ive learned that “rebuilt engine” can mean just about anything. Sometimes it means “new spark plugs”. Once it meant that they pulled the head, and honed the cylinders with the pistons still in them. The honing compound was dripping down into the oil pan. That engine wouldn’t have ran long. Another time, the engine sat in a machine shop after ”rebuild”. It was full of metal chips, and mice, and had a water pump hole drilled too deep, and into one of the cylinders. I never run an engine, from any source, until it has been completely disassembled and gone thorough by me. If a seller says he’ll “get it running” first, it’s a deal breaker. The worst thing you can do with an engine that has sat for many years is to run it. Not that I’m any genius, I just want it done right. I dont want my customers coming back to me with complaints.