I'll throw someone that most don't think of in the Experimental market and that's
G&N Aircraft. They are always at SnF and Oshkosh. They have been very easy to work with.
I tore down my IO-320, and I had a fair list of custom things I wanted like cold air sump, Precision Airmotive Eagle system, etc. I even had most of my machine work done by other local people. G&N happily took my parts, built the engine and will break it in when I'm ready.
I know you're looking for a brand new engine, I don't know what they can do for that, but it wouldn't surprise me if they will do that too. I like that they would tell me their prices of parts and if I could get it cheaper they would either match that price or gladly let me get the parts myself. Many times they would tell me to go buy things myself because they knew I could get them cheaper direct.