Hi Robert
A Lucas 5 & 3/4 headlight lens should fit in the standard rim, it may need packing with draught excluder foam to help get it in place while you insert the W clips. Then you can use BPF lamps. Paul Goff does 22/22w 6 volt halogen bulbs to fit in the UK. I've got no idea if they are availble in Australia.
Don''t be tempted to go for an LED lamp as the headlight is 6volt AC, and everyone tells me that they don't work on AC.
The electrical voltage reguation is very crude and the voltage will probably go a bit above 6volt, mine didn't last long!
Also be aware that the lights are truly awful at night.
Don