Oh...my. I think you have just found my new dash upgrade.
I believe the 993 Tach would use a square wave input signal for the tach, so it's a matter of adjusting the input signal to match. The following unit is a speedometer calibration unit, but the basic premise is the same, and this unit should work on a tach signal. You would need a dwell meter or a known-good 4cyl tach. Hook both it and the 993 up, and adjust the 993 signal to match the dwell meter. Once it's set, it's set. Note that this is a kit, and would require some electronics knowledge to assemble.
http://www.jaycar.com.au/productView.asp?ID=KC5435
Alternatively you could install a potentiometer yourself...or we could be lucky and the 993 has one already
http://www.mindspring.com/~jayk3/toyota/tachmod.htm
For the speedometer, the 993 is going to use an electronic input, which you can easily duplicate. You will, however, need a second speedometer adjustment unit as the "pulses per mile" off your bug setup is sure to be different from whatever Porsche was using. For a speedometer sensor this mount and hall-sender ring is the hottest setup I've found so far, and will be installing on my own bug:
http://www.pacificcustoms.com/mm5/me..._Code=featured
http://www.pacificcustoms.com/mm5/me..._Code=featured
http://www.pacificcustoms.com/mm5/me..._Code=featured
-Dave