The long string of numbers is usually directly above the exhaust pipe where it comes out of the engine. You might find the small video on this site interesting:
http://www.briggsandstratton.com/model-number-help/(Then select "Riding Mower Engines").
It sounds as if the previous owner has given your engine some rough treatment. Note that if it has run for a long time with the choke wired shut, there will be engine damage. The engine will have been running so rich that the oil will have been continuously washed off the cylinder wall by petrol, causing wear to the cylinder and piston rings, and some of the surplus petrol will have ended up in the oil sump, diluting the crankcase oil. You will need to change the oil right away.
