A good reference book is FAA manual AC 43.13-1B/2A (formally CAA Manual 18) "Acceptable Methods, Techniques and Practices, Aircraft Inspection, Repair & Alterations". Chapter 4, Metal Structure, Welding, and Brazing covers this type of repair. In fact anyone who is doing their own repair or better yet, building an aircraft, should have this most valuable book.
I think A/C Spruce or Sporty's may have them for sale.
Having just gone through an extensive repair due to a towing accident, I have had to straighten and replace a lot of tubing. Sometimes it's better to simply straighten a tube, and then weld in a reinforcement.
One thing I would like to add. Be careful about changing the design by adding or removing structure. Sometimes, simply adding a gusset to reinforce a joint, will in-fact transfer the stress beyond the gusset and cause a failure downstream where the addition was made.
Good Luck,