M6 x 0.75 sounds good for the plug thread, I have those taps for the thread in the head.
I have 7xxx series Ali blocks or centrifugally cast, cast iron to choose from
for the heads and valve blocks.
Stainless or silversteel for valves, I would prefer silversteel, easier to machine
in smaller dia's.
Brass for the valve seats?
Also, I am confused about the transfer port, drawing doesnt show drilling detail
to match valve seat retainers, not to mention exhaust and inlet ports.
Maybe its an age thing, or I am a page short of the book