Dave,

The hole you are talking about was added on the later model MC21's. This allowed the oil pump to feed oil directly to the right side main bearings on the crank.

You will want to block this off if possible (to not get crud/crap/dirt) down intot he crank area.

You will be okay running with out the feed

I have run 21's and 28's with this blocked off and on pre-mix with no issues.
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Charles Gallant