Undersized usually means small diameter but some people mean small load capability. If you go bigger diameter make sure there is enough clearance between the props and the hull. I'm guessing you'll need 1.5 to 2". If you are already at max dia you will probably need to get props w one more blade or greater blade area. You could call Michigan as they have very knowledgeable people on hand.
You could get a recommendation from boat owner that did'nt have it right either. Have the experts and ideally the engineers figure this out.
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