Small Engines (Lawn Mowers, etc.): lawnmower engine, briggs and stratton, stratton family

Question
I would like to replace my lawnmower engine, however I can't identify this particular engine. It has a plate on the side of the block with; Briggs and Stratton
                  Family-  YBSXS.158
                  MPR10021-01


Answer
I don't know but here is what I would do.

Go to the Briggs and Stratton web site and find the phone number.  call customer service and read them the numbers and the mower brand. They will identify it for you and tell you the proper replacement engine.  You could do by e-mail too just find the link on their page.