Experimental engines can be built by anyone out of anything.
Now, what's legal and what's prudent are 2 different things.
If you are building up a Lycoming, etc. I strongly suggest that you get close supervision from an experienced A&P who has built that model engine before.
As a matter of reference, even an A&P is not allowed to build up a certified engine without this supervision unless he has built that particular model engine before.
Yep. That why people put subaru engines and v8 engines in experimental airplanes. Theres no requirments. But like Mel said, there are dumb ways to do things and there are smart ways to do things.