I can help somewhat, Graham. First, look at the third block of numbers (the code), 79011703. These tell us that your engine was built on 17 January, 1979. (Ignore the final 03, that relates to which production line it was built on at the US Briggs factory.) Second, look at the first block (the model), 81232. These tell us that your engine is 8 cubic inches displacement (130 cc), is a second generation design (first generation was 0), has a horizontal crankshaft, has a Pulsa-Jet carburetor (the type with a fuel pump built-in), apparently has a flange mounting with ball main bearings (that is the 3 in the number, and seems quite strange for the application), and has a rewind starter. As I said, I don't really believe it has a flange mount for a gearbox around the output shaft, or ball main bearings, since it is fitted to an SB45 and they normally have neither - if you read that 3 correctly, someone may have fitted the wrong cooling air cowl to it along the way, or for some reason it happened to be fitted with an up-optioned engine, perhaps due to a shortage of the correct type. A picture of the crankcase cover where the output shaft comes out of it should show whether it has a mounting for an external gearbox. Having ball main bearings is something of an advantage provided you keep the oil clean, but it is not necessary in lawnmower service. Note that because your engine was built before 1983, it has breaker-points ignition: the Magnatron breakerless ignition came after yours was made, but can be retrofitted if it ever becomes necessary.
By using the model number (first block) together with the type number (second block) we can get the operator's manual and illustrated parts list from Briggs and Stratton:
Operators' Manual
http://bsintek.basco.com/BriggsDocumentDisplay/default.aspx?filename=lrwBF-wU7eL7hAxB91ObQIllustrated Parts List
http://bsintek.basco.com/BriggsDocumentDisplay/default.aspx?filename=Z6mmyAVJ1DajI
