shifter,
sorry mate but these twincam motors/heads ceased production around the 1983ish mark, and were never released in any locally-delivered cars, in either 70's or 80's.
So unfortunatley there are very few stockists of parts for twincam heads(18RG/2TG).. especially items like housings/timing gear..
Im not sure about Brisbane at all but ive not heard of many aftermarket parts out for the 18RG.
better off looking for a good 2nd hand item, theres usually plenty of 18RGs being sold/parted out.
www.swmotorsport.com.au have a few parts for older-style motors, could try them..