Hi Royce, welcome to the site.
Are you from the Grimsby Cadets? if so I will be down your way one Saturday soon, possibly before Christmas and I can show you how to, simply, scratch build a replacement. In the meantime I will ask in various quarters, to try and find you one. I don't build in 1/72 but I know a few who do, Lampie, jetprovost, almonkey, are you there? Was it a round, car type, or a rectangular one?
Reference the competition, it could depend on which competition it was entered into. It might also depend whether or not there is a location hole, lug, left visable, which may make it obvious that something is missing. So that's a possible other option, fill in the location hole. A small amount of PVA glue (woodworking glue), applied with a cocktail stick and levelled off using a wet finger, will dry clear. You can then paint it to match the rest of the cockpit framework. Only a real Spitfire buff would probably know and not all Spitfires had the mirror anyway. If you enter it into a cadet competition, you may loose a point (not knowing how it may be judged) but if the model is reasonably well built it wont detract from the overall model anyway. By "well built" I'm talking about seams irradicated, well painted and well decalled. Again these are thigs that I will demonstrate when I can get down to Grimsby. If your not from there than I can help via this site.
Basically I wouldn't worry too much about a missing rear view mirror, unless you intend to enter your model in one of the national shows. Do you have any pictures?
Mal