Hi Roger,
I like your design. It is similar to the Bob Shores Silver Bullet. As you probably know, the Silver Bullet fires 360 degrees apart using waste fire ignition.
I built a Silver Bullet, but changed the crank and cam to fire 180 degrees apart, as that was the sound I wanted. At the moment, I'm waste firing it with a two lead coil, a Hall sensor, and two magnets at 90 degrees driven by the camshaft. As you mentioned, one cylinder always fires into an intake stroke, but that causes no problems and the engine runs well. With the pistons moving out of phase with each other, there is little need for a crankcase breather. If the pistons were traveling together, I'd need a breather.
I'd prefer cast iron cylinder liners, or maybe steel, over the brass. You might consider a larger diameter flywheel if you like a lower rpm idle.
Unless you plan on sustained running at mid rpm or more, I'm not convinced cooling channels in the head are worth it unless you can manage more than a couple drilled passages. My Bullet has a dry head and will run idle with a few excursions to higher rpm for a good 10 minutes or so with no problems.
Roger, I doubt I've told you anything you did not already know, but I wanted to give you my thoughts. Good luck and I'll be following your build.
Chuck