I have heated and drilled out several Delrin screws. Most of the plastic can be drilled and screwdrivered out or heated to get it out of the casting. But take it easy with a propane torch- as it kills the anodized color. Then you can buy or borrow a coarse thread pipe tap that fits the threads exactly to clean the corrosion off. (3/4 inch? -I can't remember). Take your almost clean casting to the hardware store and get the right size by screwing a iron pipe nipple part way into it. (Note- a threaded pipe is conical, not cylindrical, so don't put it in very far.