I’ve been struggling to zig zag after I build up a MT is there a way to modify the d-pad on a gba sp to make it easier?

ZZMT means that you zigzag to charge the MT, not after it's already charged. You need a d-pad with a shortened nub beneath the center, be it from extended use or from filing it down yourself.

That being said, I don't recommend trying ZZMT until you have a strong grasp of the basic mechanics. This means being roughly Top 30 in the Time Trial rankings (http://mariokartplayers.com/mksc/afn.php).

Can you ZZMT using an emulator? with keyboard input?

In principle, yes.