Some of the Fox engine models haven't really changed all that much for many years now. The C frame or small case .40 is one example, I don't think there was any changes except to the carb since it first came out, The large case 40/45/46/50 engines did have a few changes along the way though. But for about 20 years the engines are the same except for the carbs. The bushing versions of the 40 and 45 had a different rod and crankpin on them. The earliest wart engines were different to, but I think it was the crankcase more than what was inside. Oh yeah, they did change the head button over time, they had high compression and lower compression versions over the years.
I see parts for the engines pop up on Ebay from time to time.