Welcome!

1) tree tick (and others useful things):
See
http://www.mariokart64.com/mksc/tips.html

and
http://www.planetnintendo.com/mariokart/mksc/advancedstrategy.html

Fast and shortly:
-shroom, release A, jump, hit something while flying, drive without pressing A, eventually press B to turn sharper.

2) Use Bowser everywhere  
Use Toad/Peach/Luigi only for the fast lap of the shortest circuits (where you drives almost all the lap in ticking mode).

3) Without using any advanced tech. Simon's has 48"60 (WR), i have 49"10 (*tR). Both using Bowser and shrooming in the middle of the lap (between wall and tree).
Some time ago i played with Toad using the shroom at the last turn and i got 50"45.
51"xx can be a good start point if you can add some techs at your strat.

termoPilucco

*tR=termoPilucco Record