Good question. The short answer is you can choose whichever keyboard input you prefer (space/wasd/arrows). Just make sure you document it at submission!
The longer answer is the hardware we will be using to evaluate the single-button input games allows us key-map specific thought patterns to any button on the keyboard (mostly). Since we have to train/set-up the hardware every time a game is run, it is simple enough to just use whichever input was coded by the developers. For instance, right now we have a Mario-like running game and have mapped onto "W" for jumping. If one of our participants wants to run instead (or say we use a different game), all I have to do is switch the input key (done from a GUI) or load in a different saved key-map.