You wont find many in competition: not competitive with modern AWD cars, too expensive for low-budget privateers and little interest in historic rally cars, at least in the US.
Saw one at the Rim of the World rally down in California three years ago:
A few more at a historic rally event when I was back in England earlier this year:
If you really want to know how to build a rally car, you should be talking to the folks on
Special Stage.