Well, avoiding any of the usual righteous comments, let's just say it is illegal to DIY-change an engine on a certified aircraft.
If you did that, you better not be trying to flog the thing openly. Anybody who is not a complete mug will look at the maintenance record and will spot a lack of a maintenance company signoff on the job.
The result is an aircraft which is worthless until somebody spends some money straightening out the paperwork.
I know changing an engine is not hard. I've seen mine done twice. But somebody who is less than clued up might bend around the oil hoses a bit too much, for example, so they split a few hundred hours later. Anyway, hoses are usually mandatory-replacement at engine replacement/overhaul. Were they changed also?