The first one or two digits in the model number are the displacement in cubic inches. There are roughly 16.4 cc per cubic inch so if your 12 cubic inch B&S is exactly 12, it is about 197 cc. The output shaft size depends on the engine size rather than output, and B&S tends to have more than one output rating for each engine size anyway, depending on what maximum engine speed they have decided on (and that is likely to be higher for horizontal crankshafts than for vertical crankshafts). For example, in the 1970s the 9 cubic inch (148 cc) vertical-crankshaft engines were 3.5 hp and ran up to 3,600 rpm. Then B&S derated them by fitting a different carburetor, and set the maximum engine speed to 3,000 rpm; since then the 9 cubic inch engines have had less than 3 hp and B&S pretty much stopped talking about their horsepower. I don't think you can assume that your engine has any more than 4 hp, and it may have a bit less.
