Oh 150cx is way harder then the same difficulty in MK64.
It took me many tries before I could beat 150cc in SMK. I think I barely got first on 150cc Special Cup.
As some of you might be aware there are codes to this game.
When selecting a racer press Y and A and that will make you shrink.
As KDR pointed out you can play the Special Cup in Time Trails.
Here is how it is done.
Go to timetrails and pick your character and on the stage screen move over to Mushroom Cup and highlight it and press L,R,L,R,L,L,R,R,A that should open Special Cup so you can practice the stages.
L and R refers to the shoulder buttons.