Have you actually had the battery load tested?
IF not, how do you know it's good.

Assuming a good battery, you might want to try removing the spark plugs and see if it cranks.
GROUND the plug wires!
There's a possibility (if a gravity flow carb) that the carb has leaked through, filling the cylinders with gasoline and hydro locking the engine.
IF that's the case, you don't want stray sparks as gasoline is spurting out.

Try checking the starter to make sure it's kicking in and out.

Briggs & Stratton engine. If it will crank over (spin) when the spark plugs are out, the valves need to be adjusted to restore the compression release to good working order.
There is a specific procedure for adjusting Briggs valves.

This is a Briggs opposed twin, so the valve adjustment isn't an issue.
461707-0145-E1

OOPS! You are right Bill, half asleep is not how to work the internet. :^(