I would come nearer thinking it may be a bad starter winding or a brush/commutator
problem in the starter. If it doesn't crank at all that would certainly be a thing
to consider. In any case the engines are easy to crank and usually easy to start
so if it is cranking slowly it could be either in the cables and connectors or in
the starter itself as well as a weak battery. Some voltage checking might clarify
this issue. How much does the voltage drop at the starter and at the battery
when you crank? If it is greatly different look at cables. The engine/chassis
ground cable is a likely suspect.
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George Downs, Bartlesville, Heart of the USA!