I have a Sabre M23 walk-behind 21" self-propelled mower, built in 1985 by John Deere. It has a Briggs & Stratton engine. It has a rectangular pleated paper filter on the right side. It is pull start, not electric, and the mower is self-propelled with a V-belt that powers the rear wheels.

Anybody know which B&S engine it has?

The model, type and code numbers should be stamped in the metal engine shroud just above the spark plug. Have a good one. Geo