Not having any bar stock large enough for milling the crossheads, I did some calculations and found that I could get a block cut from some 2" round stock. I still has a chunk of mystery brass alloy that Cedge gave me, so I cut off a 1" slice on the bandsaw and milled it into an ablong block. This was yesterday; today I decided to continue. I didn't get far.

As I was drilling one of the holes for the guides, I noticed a hot spot on the side of the block. Seems this metal gets hot when drilled, and actually melted at the bottom of the hole:

Since I also had a piece of 360 brass 2" rod, I went to Plan B and repeated the milling operation. Once I had the first block, I calculated that it was 62% of the original disc:

Here's the machining sequence for anyone that's interested. First, since the sides of the disc are reasonably straight, I chuck the disc in the vise with one side flat against the fixed jaw and mill one face flat to remove saw marks. Next I reverse the disc with the flat face on parallels and mill the other face. Now I can clamp the two flat faces on the vise jaws and mill one side (after calculating how much to cut). This flat side is then clamped flat on parallels to mill the opposite face. I now have something that looks like this:

I then use a square to clamp it vertically.

Once I had an oblong block it was a matter of
carefully drilling and milling the crosshead out of it. I didn't find the optimum milling sequence, but all was fine in the end.

Hopefully I'll finish the other one the next time in the shop.