To find technical information on that engine you appear to need B&S repair manual part no. 270962. The older repair manual, which can be accessed on line, does not cover this model. The operators manual (URL below) will give you the nominal bore (87.31 mm) but not the piston size.
http://bsintek.basco.com/BriggsDocumentDisplay/default.aspx?filename=mssAIQ-K_ny7tH217FzoPu