G'day Peter, we have minis here in Bourke which is just over the border and a long way inland. In the construction forum there was a very good discussion on the front end and lots of variations mid last year. Look up Lake Lefroy mini Explorer plans - steering assembly. While it does not give a cut this piece to length X and weld to angle Y description, the information there will let you adapt a variety of different wheels to the front end successfully. Photos show how a 20" BMX fork was cut to attach the 20mm steering shaft, the washers reduce friction between the frame head and the forks, remove up and down slop and set the correct height. The third photo illustrates the difference in fork angle to achieve the correct alignment. The similar diameter 16" bike and 400 -8 wheels are close to the same angle. The 20" wheel needs to be lifted up and back to get the steering neutral. A line taken down the steering tube to the ground should intersect a vertical line through the centre of the wheel. or thereabouts. Cutting the fork from head stem inwards on one side and from inside of the U on the other eventually gets to the correct angle and gives plenty of metal to weld to.
Good luck
Colin