Hey Todd. Pull the head and measure the bore, same with stroke. A cheaper vernier should work fine. You’re an educator so the math is pretty straight forward.
Get a hold of one of the Tag engine folks and describe what you’ve got. I’m sure one of them can get you started. I’d try Paul Kennedy in Alexandria. 703-780-3224 Obviously don’t forget to through a carb kit in it too. All the carb info should be right on it. Having the carb bore size and number might help ID it.