They are all labelled "N22"
Then they might be N channel MOSFETs, though even that's just an educated guess. One problem with the small surface mount packages is they don't have the space to put full or even half of the part numbers.
I suppose you could power the board with a 5 VDC
supply (regardless if it used USB or an older style "game port" the voltage level would be +5 VDC
) and experimentally determine, via multimeter, what this part is doing. This is a relatively uncomplicated board, so the reverse engineering shouldn't be too hard. Just record what the voltage levels on all the pins are to various combinations of buttons, one suspected transistor at a time.