3wmo19

I have a Toro 421, a real champ.  But every year I have to fool around with the engine to get it going and this year, which began with the starter rope breaking on the first pull, is no exception.  I would like to replace the engine.  The trouble is the 4 hp engine has no name plate, so I don't really know what it is.  It looks like a B&S, but I'm not sure.  It also looks like it has a gear box.  Does anybody know what this engine is and if there is a gear box, what is the ratio?