I've run into that a lot in restoring antique tractors. Many of the tires have been "filled" for weight and the calcium chloride is wicked corrosive.
Open the rusty hole up with a drill or file. If you are using tubeless, make the hole as big as the outside diameter of the valve stem. Clean the inside of the rim well. Get a body washer with a hole the same size as the original stem hole and braze or weld it to the inside of the rim. You may have to trim or shape an edge depending on the rim profile. Then just snap in a valve stem. With the valve stem in and wheel painted it isn't noticeable.