You will need to replace everything across the rear.. from hub to hub. It could be a costly replacement. If you send a couple of good clear pictures of your rear axle assembly. My son (authorized Margay dealer) may be able to get you a quote to replace the parts with new 1.250" parts. He carries 40mm and 1.250" parts. Send them to
mikimmotorsports@gmail.com
You may be able to get a 1.250 carrier for the brake rotor and for the sprocket carrier and retain the original ones you have with the kart. But the list below will give you an idea. Used may be ok as long as bolt threads are good and not stripped or parts beat to heck from use.
(2) 1.250 bearing Cassettes (3 if kart has a third bearing though it is not usually used)
(2) 1.250 bearings
(1) 1.250 brake rotor carrier
(1) 1.250 sprocket Carrier
(2) wheel hubs.
axles keys for all hubs and carriers
(1) 1.250 Axle