Building a Salzburg replica rallybug
If you want to build a replica of the Porsche Austria bugs, then it will take at least the following:
A 1971 or 1971 1302S Superbeetle or a 1973 1303S Superbeetle
A roll cage (to current FIA specifications if you're going to rally in earnest)
A limited slip differential - either a NOS or used factory item (VW or Porsche, depending on the transmission used) or something like a Quaife
A VW-Porsche 914/4 transmission if you wish to have a 5-speed
Appropriate gear ratios for your transmission - for example, MittelMotor in Germany for 914/4 gears.
Uprated coil springs for the front McPherson struts
Bilstein dampers all round, if still available
Fire extinguisher system - you don't want it to all go up in smoke if something goes wrong
A dry sump oil system, complete with thermostat and front-mounted oil cooler (behind the apron grill normally reserved for the air-conditioning condensor)
Luck - to find a pair of Solex 40PII carburettors for the early cars, or, even rarer, a pair of Weber 46IDA/2 carburettors for the later cars (the Solex were on Porsche 356, 550, 904 and 912 models, so are more common, the Weber carburettors were on the 904 Carrera GTS race car, so very rare)
Along with the above, there are lots of other modifications needed, some of which can be found within this site - different camshaft, cylinder head work, valves, fan shroud modification, 5½ x 15 wheels, appropriate M&S tires. The original cars appear to have over-sized pistons and cylinders fitted to get them closer to the capacity limit (85.9mm instead of 85.5mm, to give 1599cc instead of 1584cc)
Alternatively, you can look through the various rally championships in your country and build a Rallybug to the specifications allowed - 2.5-liter rally car, anyone?