If you use a stock type gear, the only way to adjust the cam would be to "jump" a tooth. With an adjustable gear set, you have different keyways on the crank gear for the desired alignment. Some sets also use degree bushings to align the gears.

The long and short is that you could use stock gears if you could find a way to guarantee that the setting you want can be maintained. There is nothig special about the adjustable gears themselves, just the alignment features.