Tub drain rusting out

I have a 1930's home with a tub in the upstairs bath. There is heavy rust around the drain and I am afraid it will soon begin to leak. Otherwise, the tub is in good shape and would be a problem to replace because of the stairway.
My idea was to cut the existing drain and rust out which would leave a 2.5-3 in. hole and try to find a drain to cover this opening. Is there a drain, residential or commercial available to do this?

I am not aware of anything made for such a repair. A tub drain shoe is unlike other common drain applications, so anything you could do would be like the work of Frankenstein.

Most drains are brass, so if there is rust, it is probably the cast iron. Once that starts, you're not likely to be able to do much about it except replace the tub. If it was something special, you could take it out and find a place that sandblasts it and then reapplies a coat of porcelain, but this often costs more than just buying a new tub. Any inplace resurfacing is suspect when you already have rust, and not much better if you don't. If it is truly just the drain, you could replace it. But if the tub, bite the bullet and replace and remodel that area.

IF the tub is rusting so the opening is getting larger, you would have to have an enameled steel tub. If so there is NO repair possible. The tub has to be replaced, but I am not sure what having "stairs" has to do with making a replacement impossible. Enameled steel tubs are "useless" the minute they are installed, and only go downhill from there. WHich is why I will only install them if the customer furnishes it.