[Marshall V. Daut]

After all the usual suspects have been eliminated (timing, carburetor, fuel flow, points, etc.), make sure the muffler isn't clogged. If it is, the engine will still idle nicely and accelerate smoothly up to about 15 mph, and then it will go no faster because the exhaust can't exit fast enough. Cheaper repo mufflers from the 1970's to the early 2000's (i.e., Midas mufflers, the only game in town at the time) tend to rust inside, so if the engine backfires or time takes its toll, the front baffle breaks loose from its spot welds and pushes toward the back. This effectively blocks most of the exhaust flow, which backs up inside the cylinders = low power. Try a known good muffler and see if that solves your problem.
Incidentally, I wrote an article about this very same condition in the printed version of "Model A Trader" in the late 1990's in the my column "What's Wrong With It Now?". Maybe it can be pulled up on-line, or I can scan it. The article describes what one Model A repair shop owner went through in tracking down the cause of the same problem you are experiencing. Yup - it turned out to be the cheap Midas muffler from the 1970's.
Marshall