After a dab of green LocTite (680) and letting it cure overnight, I sat down to start the engine, with the idea of letting it run once a day for a while. It simply would not start. While figuring out what was wrong, I tried tightening the screws to reduce the wobble as the piston moved, but they were already tight. I found the batteries in my electronic ignition needed to be replaced, and the lower level of fuel in the tank made positioning it difficult. It eventually did turn over a few times, but wouldn't keep running. The vibration was so bad that the carburetor throttle lever would bounce out of the range it works over.
Last week
@Steamchick had mentioned a triangular brace on the other side of the head, so I started thinking about that. The first thing I did was pull the drawings back up - this is on sheet 4 of Webster's plans. He left out all hidden lines on the head "for clarity" (a note in the top left). I started adding the hidden lines so I could see where everything was. The top of the head is probably the best place to put a reinforcement, since the force from piston pushing on the head produces the most torque farthest from the base, but there's no more than 1/4" of wall up there to drill and tap without breaching the combustion chamber.
I eventually added two #8-32 screws in the base, on the opposite side of the 6-32 screws that hold it to the side frame. Adding in the hidden lines allowed me to see the screws coming in from that side would hit the #10 screw that holds it to the base. I displaced them "forward" (farther from the flywheel), which looks to the right in the view on the plans. So I redrew the head from the other side (where the cylinder used to be) to show the way they look from the side looking at the engine. This screen capture off my computer should help show it. Dimensions in black are all mine, the bottom half is all mine, the notes in black are mine.
The bottom half shows the bracket that would be flush with the front of the head, adds two screws on the left, 0.160 from the end, and two screws in the bottom. The head is shown with bracket copied onto it on the right.
A little search around the shop led me to a piece of scrap the same thickness as the side frame (5/16" or 0.313"). Coincidentally, almost exactly the distance from the edge of the head to the edge of the base plate. The scrap had an odd shape, but I just needed to trim and square it up a little to have something usable. A second search led me to find exactly two screws for each of the two places. That meant I had what I needed to clean up the scrap and make a brace. I spent some time trying to figure out how to do this without disassembling the entire engine. I figured I'd clamp that bracket to the head and use it as a guide to drill and tap the new #8 holes. Once the holes were in the side of the head, I could pull the bracket off, move the engine over to the CNC mill, zero on the vertical line where the new screw holes are, use the 1.586" calculated distance from there to the first, new vertical hole, and drill through the base plate.
It actually all went together easily. Pay no attention to the fact that the two 8-32 screws on the side holding the new bracket don't match. One isn't stainless. That's just cosmetic.
By the time I finished this, I had used up the afternoon, so I don't even know if it works yet. I think that curved surface on the top of that bracket or brace or whatever you call it needs some more finish work