You probably can't do it with USB Loader GX, USB loaders have no access to change that since controller input is handled by the IOS, and Fakemote doesn't support GC controllers.
In some cases there might be cheat codes available to remap certain buttons but they are highly game specific.
But there is one thing that might help: RVLoader.
It's designed for portable Wiis, but it has the ability to emulate Wiimote controls using a GameCube controller, and allows you to remap the controls. It would replace USB Loader GX (at least for that one game) since it has its own USB loading functionality.
Edit: You are in luck, there is just such a code for Mario Kart Wii.
For RMCE01 (USA), paste in txtcodes\RMCE01.txt on the same storage as your games (everything up to and including the 60000000 00000000 line), edit the XXXX and YYYY values to your liking and then go to the game settings in USB Loader GX to enable the cheat:
Code:
RMCE01
Mario Kart Wii
Key remapper for GC controller [mdmwii]
C21afb58 00000005
39C0XXXX 7DCF0038
7C0F7000 40A20010
7C0F0050 39E0YYYY
7C007A14 B0040000
60000000 00000000
XXXX = key you want to change
YYYY = key assigned
Values:
0081 = D-Pad Left
0082 = D-Pad Right
0084 = D-Pad Down
0088 = D-Pad Up
0090 = Z
00A0 = R
00C0 = L
0180 = A
0280 = B
0480 = X
0880 = Y
1080 = Start
For RMCP01 (EUR), paste in txtcodes\RMCP01.txt on the same storage as your games (everything up to and including the 60000000 00000000 line), edit the XXXX and YYYY values to your liking and then go to the game settings in USB Loader GX to enable the cheat:
Code:
RMCP01
Mario Kart Wii
Key remapper for GC controller [mdmwii]
C21afbf8 00000005
39C0XXXX 7DCF0038
7C0F7000 40A20010
7C0F0050 39E0YYYY
7C007A14 B0040000
60000000 00000000
XXXX = key you want to change
YYYY = key assigned
Values:
0081 = D-Pad Left
0082 = D-Pad Right
0084 = D-Pad Down
0088 = D-Pad Up
0090 = Z
00A0 = R
00C0 = L
0180 = A
0280 = B
0480 = X
0880 = Y
1080 = Start
In your case, you should change
39C0XXXX -> 39C00088 and
39E0YYYY -> 39E00880.