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I disassembled my core engine, farmed out some inspection work, sourced parts.
I was tempted to assemble it myself, but in the end I got a really good price from an engine shop to assemble my parts and run it on the stand.
If you can find a core / mid-time engine where they will guarantee that the case and crank will yellow tag that would be great. My crank failed inspection.
You need to be knowledgeable - or ask ... there are some gotchas out there. Some dash numbers of Lycomings had carbs mounted in funny spots (that won't work on a 7A for example), but you may be able to swap an oil sump and fix that issue. You need to know if the core is a hollow crank (can be setup for CS prop) or a solid crank (can't). There's things like helicopter engines, etc., etc. Don't limit yourself to looking for an o-360-a1a (for example) that'll limit you to a small set of engines. Instead figure out if that -z9q can be made to work. Oh, also beware that some Lycomings for twins are setup for contra rotation. But the information is out there. Previously mentioned overhaul manuals, parts manuals, Wikipedia, these forums, your EAA chapter, or even call Lycoming.
In my past I've overhauled a Corvair and a water cooled VW engine.
People have been doing this all along, but I'd say since pre-punched came along it has fallen by the wayside. About the same time as all new avionics, all glass cockpits, real leather interiors, etc. became the norm.
And yes, I toyed with the automotive engine idea for quite a while. After reading multiple incident reports where the home-brew water lines burst, resulting in off-field landings, that idea was shelved.
Lastly... one more idea --- find a partner and co-owner! Lots of people want an RV but can't commit to the time or total cost of building / owning.