yeah you can easily bypass the starter switch in the ignition lock, just cut red/black wire to ign switch or the red/black wire behind the fusebox, job done.
In fact later mk3s have a 2 pin plug in the starter wire from fusebox as factory, its for alarm equipped models and automatics. could get an auto plug, chop off the feed side wire and run the output side to your starter switch, job done.
what I would do in this situation, is pop the red/black wire out of the ignition swith. next I would go down the scrappy, and find a mk3, pop the starter plug off and chop off a really good length of the red black wire. next I woudl wire in a relay next to the starter, run the ouput pin of the relay to the length of wire you cut out, and pop that on your starter. tape up the old plug to stop water getting in and damaging the pin.
run the live and earth to relay direct to the battery, with an inline fuse for safety. then run the relay trigger wire to your push button. use an ign live feed for the input side of the switch, so starter will only turn when the ignition is on.
that way all the old wiring is unmodified, and its dead easy to put back in case your push button or relay fails. plus you have a nice relay there to ensure the lots of power to the starter, and you also can use whatever push button you like as you aren't running the full starter power thru the switch
