Boy, that is a problem. I suppose you could try it with just the 6, but it doesn't seem like a good idea. You might need to go with a clam shell hub adapter. I don't know what you have available in Oz, but here in the states we have ones made by Sportscarpat and the Manic Mechanic.
Another way around the problem would be to use a 16" moped wheel with both a pedal sprocket and an engine driven sprocket. With tire it comes out to the same size as a 20" tire. Let us know how it works out.
SB