Because the engines and transmissions are very different, the frame is different for the powertrains as well. If you are asking about swapping out your rear drum brakes and install rear disc brakes, it is not exactly straight forward. Disc brakes are actually combination disc & drum - disc for typical braking, drum for parking/emergency brakes. As such, your parking brake cables would have to be changed to work with the combination disc/drum brake setup. You would need to see if the rear hubs are identical - I think they are between the AT200 & ST204, but not certain. If not, you would need to put ST204 hubs on your AT200. Your proportioning valve that impacts brake fluid pressure differences between front and back would need to be changed as well as disc brakes have a different pressure target for rears vs. drum. Doable - yes. Is there benefit for the cost and effort - very little if any.