Thanks, folks, for the comments and support. A five cylinder version may be in my future sometime. I've thought about building Rudy Kouhoupt's 5 cylinder radial using my slave valve arrangement and may do that at some point.
I just wanted to add a few notes for those who are interested.
The crankshaft on my model has a diameter of .1875" of an inch. I think Nick's crankshaft was closer to .27" so larger crankshafts are certainly not a problem.
Another thing to consider is the air pressure it takes to run my engine. While the spring loaded ball exhaust valve engine is somewhat simpler to make it does require from 15 - 20 PSI to run. And, it won't idle as slowly Nick's design.
Finally, the parameters of the spring and ball bearing are somewhat critical. The ball bearing must move freely in the bore and a clearance of several thousands of an inch is desirable. The spring must also fit loosely and have a very small compression force. The springs on my engine are about .008" wire size, 1/8" OD, and just barely long enough to hold the ball bearing in position between power strokes. The stiffer the spring, the more air pressure it takes to run the engine and the faster it will run, but won't idle as slow.
Chuck