If it were me, I'd probably leave it as is. If I were determined to change it to thru axle, I would use the round thru axle dropouts from Paragon. I would use the current axle slot to center the correct size hole saw and cut out a round section of the existing dropout. You can then fixture the new round thru axle dropouts and weld them into the cutouts.
DR2065: Stainless Steel 1-1/2" OD x 1.1" Rear, 12 mm Hub