It is possible to make your own heads (out of billet aluminum) - if you can produce 3D CAD models, can generate the toolpaths and have full CNC milling machine equipment.
You have to make them as a "clamshell" design (two halves) - with water jackets milled out of the interiors, then use an o-ring material to seal all the various stud and plug hole locations between the two "sides" as well as around the exterior. We designed all our own heads for the FlatCad engine (blown flathead Cadillac) - is a lot of time and money (and we had our own CNC equipment).
If you have those capabilities, the $$$ it takes to buy 4 big chunks of 6061 aluminum and access to the equipment, let me know . . . might be able to collaborate on a "winter project"! LOL
Here are some pics of the FlatCad heads:
Heads-006.jpg
Heads-007.jpg
Heads-005.jpg