I mostly agree with Gary Skeens, having repaired steel tube fuselages that were bent much worse than this. I'm not sure you would have to jig the fuselage in this case--based on the photos, it seems the damage is pretty well isolated to the left landing gear attach area.
The first step, as Gary says, is to remove the paint or powder coat all around the affected area. If it is paint, the easiest method may depend on the capabilities you have. Sandblasting is good, but a good chemical paint stripper may be the easiest.
Next, I would cut out the two vertical pieces of tubing that are buckled in compression. It may also be necessary to cut out the diagonal brace in that bay, but I would try to avoid that, since the diagonal establishes the location of the bottom engine mount at the firewall.
If it were me, I would want to avoid having to rebuild the lower longeron, the rudder pedal attach area, and the steel plate reinforcements. Therefore, my approach would be to try and straighten the lower longeron.
I would use a portable hydraulic frame straightening tool such as those sometimes used in automobile frame straightening--in the U.S. the most common brand name is Port-A-Power--you can look it up on the Internet to see what it looks like.
My suggestion would be to first temporarily grind and reweld the crack on the longeron at the firewall.
You would attach spacer blocks (perhaps 50 mm thick or so) underneath the longeron at the firewall and at the forward lower wing spar attach point. Then you would attach a strong steel bar across the two spacers, spanning the length of the damaged longeron area. (Better yet, weld the spacer blocks to the bar to make it easier to handle.) The steel bar would then provide a strong surface to pull against using the Port-A-Power. I would suggest using a steel block against the top of the longeron at the bend to distribute the Port-A-Power force so as not to distort the longeron.
You will need to use lots of heat on the lower longeron, the steel reinforcing plates, and other associated structure during the straightening operation. I would want to have at least three people present--two people using oxy-acetylene torches with large heating tips (we in the U.S. call them "rosebud" tips) and the third person operating the Port-A-Power. You would need to get the entire area (including the entire length of the longeron, the steel reinforcing plates, and the rudder pedal attach crossmember) up to a dull cherry red before attempting to pull the lower longeron back straight.
Once straight, I would perform a dye penetrant inspection of the entire area, looking for cracks. If none were found, the next step would be to replace the vertical members that were previously cut out.
Last, I would reinforce the repaired lower longeron crack at the firewall by forming and welding a "finger patch" over the cracked area, extending upward onto the vertical member and aft along the top of the longeron. FAA Advisory Circular AC43.13-1B (available on the FAA website) has details of finger patches.
If this method does not work, the only recourse would be to cut out the lower longeron in that bay and rebuild all the associated structure. Here again, AC43.13-1B has information on how to do so.
I hope this helps!
G. Michael Huffman
SportAviationSpecialties dot com