There's a product called redlyme that is supposed to be real good for cleaning out cooling pasages on the raw water side of heat exchangers & raw water cooled motors. It's diluted 1:1 & costs 30 bucks a gallon. You'll need one gallon for the MD7A. Hardest part of this operation is getting the exhaust hose off the muffler.
Apex - Rydlyme is the world's leading biodegradable descaler, rydlyme is designedto dissolve water scale, lime deposits & rust deposits, but is not water treatment orwater conditioner.
Cas026, If your motor isn't pumping water check the gasket on the strainer first. Getting to the raw water pump is a real PIA