How can I repair a pasta maker that was washed with water and has started to rust?
I'd start by making up a batch of sacrificial pasta that you can run through the roller and the cutters. This will pick up any remaining moisture, remove any rust particles that the cutters knock off each other, and give you a better idea of how bad the damage is.

If the cutters still cut cleanly and the damage is mostly cosmetic, that's good news. You could try to remove rust by carefully applying something like a non-scratch Scotch-Brite scouring pad, but I'd avoid abrasives like steel wool. You want to be careful to avoid making the problem any worse than it is.

If the cutters no longer cut cleanly, it may be time to look for replacement parts. If you can locate the manufacturer, that's your best bet for getting proper parts. You might even be able to send the machine back to be refurbished. Check the instruction manual, if you have it, for an address, phone number, whatever might help. If that's not an option because the manufacturer is out of business or whatever, look for similar machines that you can use for parts. Maybe you'll find a Domus machine on eBay that has a good set of cutters. Carefully measure the width of the cutters and check other machines -- it may be that the cutting rollers from an Atlas or Imperia machine will fit into your Domus.