Based upon the model# and serial# info you provided, the engine is a Briggs 12 cubic inch "Quantum" built in 1997, so 18 years old. Was one of Briggs' best engines, although somewhat dated technically as a flat head (L-head).

Your troubleshooting is starting in the right places, but is somewhat scatter shot. Not certain whether you have ignition problem or not. Before you jump into the carburetor and other areas, you need to systematically troubleshoot the ignition, then the fuel system, and then if necessary go into the engine and look at the valves, head gasket, etc.

Start with ignition. Remove the sparkplug. With tape or string tie the "deadman" or operator presence bail in the run position. Then hold the spark plug wire with one hand and pull gently on the starter cord. You should feel a jolt. If you pull aggressively you'll get a substantially stronger spark. If no spark, remove the blower housing and disconnect the ground wire on the back side of the coil. Repeat the test and see if you get a spark (or shock). If you now have ignition, the problem is with the operator presence system and needs to be trouble shot. If still no spark, you probably have a bad coil, even if you say you replaced it with a "known good" coil.

Once you get the ignition fixed, and it still won't start. Test the engine with a squirt of "starting fluid" to see if it briefly runs. If it does run, the problem is in the carburetor or fuel system. Remove the hose from the tank to carburetor and see if gas is flowing. Then remove and disassemble carb. Presumably there is either varnish deposits from old gas, or a powdery substance caused by water damage. Either will need to be cleaned, including the jets and float needle, or the carb replaced entirely.

If this still doesn't fix things, it might be time to look at replacing the engine. At 17 or 18 years old, it may not be economical to go further with the existing engine. A new Quantum should be available on-line for between $150 and $200. Bear in mind that there were thousands of model permutations, so make sure you get the right one in terms of crankshaft length and diameter, throttle and governor connections, deadman controls, etc.

I would've posted this earlier but I was trying to get a successful repair before I left for holiday. I think my original post was more haphazard than my repair lol..I was rushing as time was running out.

I pushed the mower into a dark area and found there was no spark! With a new armature and spark plug I decided to check the stop switch connection as it didn't seem correct. Does anyone have any photos of how to connect that wire as there is a plastic lug. To me there were two options, connected or disconnected. I then got a spark but no start. I then switched to the fuel side of things and I had to clean up the carb and ensure all mechanisms were free....they were pretty sluggish to be honest. The choke was not opening up correctly so cleaned it all up made a slight adjustment and hooray the trusty old beauty fired up.

The old armature was indeed dead but I then made a mess of the stop switch connection which caused a red herring. Entirely my fault as I was rushing. If someone could show or tell me how to connect the stop switch to the ground I would be very grateful. I am away on holiday for 4 days now but I am keen to get old faithful working when I return.

Massive thank you to ggoyeneche for the excellent reply.

Changy