easiest way is to join the local rally club ask questerns there,some people here wont know.

what you need is all class dependant,eg weld in etc

you may be able to purchase the correct book to base your build on

but ill have a stab here
1.weld in cage to spec with tig
2.fia approaved fixed back seats
3.gutted flamables in side car
4.fire extingusher
5.make sure the car has no oil leaks etc make sure it mechanical sound
6.proper rally tyres
7.if still avaiable group b rego