Thanks Richard, that is a 6 cubic inch (100 cc) engine of the second design generation, with a horizontal crankshaft, Pulsa-Jet carburetor, plain main bearings and a rewind starter. It was made on 25 April 1968 - your mower was probably made just a few months later.

Here is the illustrated parts list for your engine:
http://bsintek.basco.com/BriggsDocumentDisplay/Z6jmsHVJ1DajI.pdf

When you start your thread and post pictures, our SB45 guru, Deejay, will be able to help you assess the machine's condition and what you might want to do about it.