Ok, the fix is complicated and requires some specialty in the field of welding, heat treatment and machining.
First, you will want to buy some 4130 tube from performance metals in the Sydney. Tube should be heavy walled that can be machined to fit snugly up into the downtube from the bottom and extend past the damage to up around the head tube joint.
You will need to cut out the engine mount spacer tubes (cut them out with gas from the bottom and leave the upper and lower welded bosses in the frame - more to follow)
Mount the frame onto a large lathe saddle and run a boring bar down the length of the downtube to cut the seam weld excess off for said snug fitment of doubler tube.
A few 8-10mm holes are drilled into the existing outer tube to allow for stich welding of the inner tube after fitment of doubler (this serves to pin the inner repair section to the outer damaged tube, new engine mount spacer tubes are turned to inside diameter of 8mm same as stock and approx 2mm wall thickness.
Drill and ream the lower portion of the tube (oversize the existing bosses to accept the fabricated spacer inserts and continue to bore through the inner tube to make a new hole for the spacer inserts to slide through) the existing bosses are used as a guide or pilot for your drill and reamer.
Weld from the bottom to secure the spacers (upper spacer first so you dont obscure access by the lower spacer), the bushes should also have a 45 degree bevel cut on each end and the finished oversized hole should be cut with a 90 degree c/s to prepare the inner and outer spacers for a final weld in four places around the end of each and then file the bead flat to restore the original dimension.
Obviously you can trim out the rust damaged section of the original tube before the installing the repair section.
Now you need to bend a new gusset, use a section of 4130 tube to either brace the corner as a straight notched piece (unbent option) and then plate the sides in to close and add stiffness- or bend the 4130 to match the original contour and plate in the sides as said.
The original design is a die forged plate section which is going to be difficult to replicate although possible with sheet metal skills. I think the tube gusset left open would suffice and not become a moisture trap.
All welding should be carried out by competent persons and heat treating the entire frame after repair to normalise and stress relieve is highly recommended.
Since you are going to this length, consider a magnetic particle test over the entire frame to identify any cracks which should be repaired prior to heat treatment.
I consider this a good as new permanent repair, but it will cost you. I can mag particle test your frame at no charge, I think heat treatment for the frame runs at about $200 and the tube is around $50 plus freight. Machining and welding is per the hour and Niccom at Loganholme is very good and specialises in frame repairs, the fellow mentioned at slacks creek is also of good repute, but I have no experience with him.
Food for thought
