Players with motor impairments each have their own unique requirements, often met through unique and specialised combinations of hardware. However these combinations are fairly standardised, through tech such as accessibility switches (famously used by Stephen Hawking), which are simple binary input devices such as blink detectors or microswitches that map to buttons or keypresses, or eye tracking, which maps directly to analogue mouse position/movements.

Being reliant on these technologies often also means only being able to execute a very small number of inputs, so the more you are able to simplify the controls, the more of these kind of setups will be compatible. For example if you’re making a one button endless runner, it will then compatible with everything from a wheelchair mounted button to an eye-tracking blink to a sip-puff tube.

Best practice example: Forza 4 (video)
Best practice example: Day of the Tentacle (video)
Best practice example: Minigolf Accessible
More information: Switch access on Wikipedia