I think the '18 is the best engine. not only due to its better porting but its crank and crank cases design.

As for stock pipes, MC18 stock pipes make peak at around 10,800rpm. Stock '21 pipes make peak at 11,000.

If your fitting the motor in a '21 chassis I would go for '21 pipes, if for nothing else but ease of fitting.
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If I have to take the carbs off once more...