I can't think of a way to tell, Deejay, other than asking finbar to take some pictures of numbers cast into the crankcase, and then asking members who are hip-deep in Briggs engines to compare the numbers with various engines they have.
I think it is not all that likely the engine was replaced, since the "new" cowl is not from a remotely suitable engine. More likely the original rewind starter clagged out and the previous tenant just reached for the nearest available replacement starter that could be fitted with the least effort. If so, it is still the original engine, just with the wrong cowl.
