I use 1/2 inch mild steel as per original Jonesy. Lathe the 3 grooves for the rear and face of the ends.( plain for model 1 and single groove for model 2 with tin wheels. Same for the fronts. Grind the old welds and snap em out, redress the axle mounts, flush the new ones and set up with a 45 magnet and weld em in. Simples.
