Well I thought about this a bit, & thought if I was using my Harrison chassis I'd want the rest of the car as Australian as possible so
Option 1; 4.5 lt Repco Brabham ( the old formula Libre engine made for sports car racing) with a Harrop g'box behind it.
Option 2; this would have to be the trusty ol' Ford Cosworth 3 lt & a harrop box as well.
Like to idea of option 1 though. A very long time ago I saw one of those engines in an FJ show car.......amazing
Rog