The gauge is 3.5", but the scale is .75"/foot. Assuming that your track gauge is 5", then you'd need to scale everything that depends on the gauge by 1.43. However, you can scale anything that's not dependent on track gauge by a different factor. I'm building the A3 in 1.5" scale, meaning I can double most parts' dimensions. But anything that is transverse to the frame needs to be recalculated. Things like fastener sizes, rod diameters, etc. can be adjusted pretty freely.
There are castings for the A3 drivers and cylinders in both scales; for 5" gauge you might want to get castings locally for a different engine unless you want to machine to cast your own. The shays wouldn't have that type of problem.