The only solution to this era of Beetle dash is to buy a cap for it.  The whole dash pad for these is not aftermarketed and when it was it was rather expensive.  If you can't swing the cap there are flexible sillicone based fillers which can be used to fill the cracks.  You can follow up by using Marhyde vinyl paint to give it a new flexible black surface.  It will not look as professional as the cap but it will be an improvement over a cracked dash pad.