I think I'd take several older engines (pick your favorite) and only modernize a couple of things... Fuel management and metallurgy/production tolerances.
I could probably safely double the output of my Bug's engine with just those changes while making it live longer. Well OK, I'd use more modern heads (Dual ports didn't start production until '68) and I'd need an oil filter that will screen out more than just boulders and small children.
You could do similarly with a great many engines from the past. I'd love to see what one could do with the Vega Cosworth engine, the powerplant from my old BMW 2002 and a 240Z with those kinds of changes.