Here's a suggestion....
Find a pair of Big bike or a like rear sets (lft side equipped w/shft linkage) from a Cycle Salvage yard. Go to OSH or alike and find the hardware that you'll need to mount them.
-Measure and cut a 1" wd x .125" (1/4") thk steel bar stock to some length.....I like to cut up cardboard and use it as a template and go from there
-Ensure you leave enough length of the bar stock to mount the rear-sets.
-Drill 2 thru holes to the equiv distance & centered to bar stock, of the existing peg mounting studs.
-Mount the plate to chassis to ensure a proper mount/fit.
-Measure and drill the thru holes or tap holes (SAE or Metric) just above the plate mounting holes to mount the rear-sets. Be sure to measure for clearance.
-Mount the new rear-sets to the plate
OSH may carry rods that you could use to complete your shift linkage. Or if ya know a welder, he maybe to help fabricate something.
-After a successful mod. Remove the plates and paint or powder coat you favorite color.
NOTE: I don't own an RX-12, but am sure this idea would work. And fwiw, you don't want the pegs too far behind you = bad/uncomfortable riders position.
GL and hope this info is useful