How do you set the timing on a 14.5 ohv Briggs and Stratton?

There is no way to adjust the timing. As long as the keyway (little square aluminum wedge) is in place between the flywheel and the crankshaft it should be in time. If the keyway is sheared or even damaged it will make the engine run rough or it won't run at all.

If you remove the nut holding the flywheel onto the crankshaft you will be able to see the grooves in the crank and the flywheel that this keyway slides into. If those two grooves are not perfectly in line then that is your problem.