The timing is not much of an issue. The ignition timing will inevitably be correct if the flywheel key is intact and in place correctly. The valve timing requires that you put the camshaft in the correct rotational position when you reinstall it. Briggs engines have timing marks on both timing gears: the one on the crankshaft, and the one on the camshaft. You position the crankshaft so that the dot on its gear is at its nearest point to the camshaft, then you slide the camshaft in so that its dot will end up at its closest possible point to the crankshaft gear's dot. Note that the gears are helical, so the camshaft will rotate a bit as the gear slides into mesh: you have to start with the dot about one tooth below the crankshaft one, because the camshaft will rotate clockwise about one tooth-width as it slides into place. It's a bit like leading a duck slightly when you point a shotgun at it. (Many Hondas don't have a dot on the crankshaft gear, because it is not on a key, it is just pressed on. You use a slightly different method with those - you put the crankshaft on Top Dead Center, then slide the camshaft in so that its dot is as close as possible to the crankshaft gear. I prefer to put a centerpunch dot on the crankshaft gear before I lift out the camshaft, so I don't have the hassle.)
Installing the piston and rod is a bit more complicated, since you have to worry about half a dozen things while you are doing it. This would be a rather long post if I went into all of them.
