These old speed parts have become quite rare and pricey. There are manufactures that are remaking them today but are still very pricey.
Here is a link to an article by Hot Rod Magazine that talks a little more about some of the options available to hop up a banger:
This is a short video of a hopped up bannger tearing it up a hill climb event in Europe. Not too shabby for an engine that originally made 40hp.
They sound great when they are winding out. A very unique sound to them.