I'm trying really hard to thing of a single bike I can recall that has a cast iron head. Not a single moped off the top of my head. Typically you want alu because it sheds heat faster.
I would just turn down a stock vespa head, or run a decompless head and mill the cylinder down vs. the head.
Or, you could likewise start with a stock euro top end and be able to do much of what you're looking to do without milling/welding anything.