A gentleman from down under built a 924 rally car with a rotary. It required major modifications to the crossmember to fit it among others. He's over on the 924board.org.
Personally, I'd not bother, just buy Mazda's copy of the 924/944 and get a 2nd-gen RX. The Rotary is a bit of a torqueless wonder (though they do sound good with a PP) and if the 2.5 is in good shape, run it until it dies. If you want more power, put a V8 in it.
924Board.org :: View topic - Project RX924 begins!