Don't recall ever hearing about a ready made longer swing arm that will slip into place. You will have to research what is out there & determine what would require the least amount of modification to fit.
In the 70's when a lot of dirt bike suspension experimentation took place, there were some aftermarket kits available for certain popular bikes to perform a "lay down" shock modification. These kits usually consisted of bracketry to relocate the top shock mounts forward of the OEM location & reinforce the new mounting location. The kits would often include a 1" or so section of tubing/slugs & reinforcing plates to lengthen the swing arm. The swing arm would be cut in half, then reassembled with the slugs installed into the cut section, then welded into place. The reinforcing plates were then run along the top of the swing arm tubing on each side & welded into place. Swing arms in those days were usually made from round steel tubing. Probably a bit more tricky to try the same thing with square or rectangular tubing.
J.
