you wont be able to hack the analog controls for the N64. I think Happ sells an analog joystick that you might be able to adapt with a lot of work, but they are like $200 A PIECE!!.
Basically.. you can hack the d-pad and buttons on the N64 joy with little problems and play any games that use those controls. I forget if Mario Kart can use the D-Pad, but even if it does, it's not the way to play the game. sorry.