Regularly on ebay you can purchase portholes for Williams F-7s. I just measured one and they appear to be the same size or a hair larger. If all else fails that should solve your problem.
Yes, and many/most "portholes" will probably do. The look and contour may or may not be different. I wouldn't care, most likely. It's just a dirty old diesel loco.
I have also used a product called "Sobo" or "SOBO" glue to make headlight lenses, portholes, windows. It is/was available at HobLob, etc. I think that it is for fabrics. Looks like plain white glue out of the bottle, but it is not. Pretty tough. Using a toothpick, get a blob on the end and start going round and round until it grabs all over and closes the hole (practice on something else, first). If it looks too thin, just add another drop or two (no twirling). Let it cure a day or two. Sometimes it will look milky for a while, especially in cooler temps, but that will go away.
Obviously it has a workable limit relative to the hole size. Learned about Sobo from an HO guy, but I've had luck with it.
I guess that it is a good glue, too - never used it that way!