Brent: More horsepower = more fun!
The original engine, at something less than forty horsepower for takeoff, becomes marginal in hot weather with a full load including gas and two passengers. Any additional horsepower translates into better takeoff and climb performance.
With a propeller, one cannot "shift gears" to get more power. Generalizing, most of the propellers are pitched to allow something around 2000 rpm for takeoff. The builder could select less pitch when picking a propeller but that in turn limits the maximum flight speed by allowing the engine to turn faster than it should.
All of the Model A Pietenpols that I have seen use a higher compression head of some sort. Of course, that translates into more horsepower. Generally, the builders have limited the compression ratio to 6:1, in line with what most Model A owners feel is "safe". Most of the H-C heads I have seen have been aluminum, which is wise because weight must be considered when building an airplane.
Your idea of using a "touring" engine is perfect. Add a Thomas 'two plugs per cylinder' aluminum head, two mags, and you would have an engine that would be ideal. Gar Williams
