I'm guessing you have a cast iron tub. If it is rusted out around the drain, the only good fix is to replace it. You might get some more service life out of it with some epoxy, but it wouldn't look great. See what the pros have to say... If it is just surface rust, you might be able to clean it off, then apply some paint after rust conversion, then reset the drain. You'd likely require acceess from either one end, side or beneath to get to it.