You caused the oil leak by tilting it up with the carburetor side down, which you should never do. Sometimes oil is sucked into the cylinder and results in a hydraulic lock. Take the spark plug and air filter out. If it's a pleated-paper filter, you'll need to get a new one if the mower can be used again. Pull the starter cord with the plug out to get a feel for the severity of the problem and to clear the oil from the combustion chamber. Now you can tilt it up with the carb side high to check for a bent crankshaft. Spin the blade to see if the blade bolt head seems to describe a small circle rather than just turning in place. The other problem usually resulting from an accident like this is the flywheel probably sheared off the key and tuned on the shaft. You'll most likely have to reset the flywheel before it will run. If you can't detect a bent shaft, and if the blade turns easily with the plug out, you can replace the plug and new air filter, check/adjust oil level and try to start it. If it kicks back and backfires, etc., you'll have to reset the flywheel with a new 'Woodruff key'.

I had this exact same problem after the gas tank ran dry, and I cleaned the carburetor as follows: 1. Turn off engine key and disconnect spark plug. 2. Pump out gas tank (or tilt mower) to gas can. 3. Remove air filter cover (1-bolt, 5/16" socket) and air filter. 4. Remove 3 air filter housing bolts (5/16" socket). 5. Remove air filter housing from hose in back. 6. Carefully remove carburetor gasket from carburetor. 7. Slide fuel line hose clamp 1" up hose (pliers, channel locks) but do not remove hose at this time. 8. Remove two carburetor bolts (3/8" socket). 9. Rotate carburetor CCW to remove throttle cable. Raise the carburetor, remove fuel line from the carb and hook fuel line around the engine key (to keep residual gas from draining). Empty gas from carburetor bowl through inlet into gas can. Blow dirt off of the carburetor with compressed air if you have it. Thoroughly clean the outside of the carburetor with Gumout Carb + Choke Cleaner or equivalent spray cleaner. Remove the bowl nut (1/2" socket) and remove bowl. Thoroughly clean bowl (there will probably be some grit on the bottom) and set it aside. Insert tube in cleaner nozzle and thoroughly spray the aspiration tube and fuel inlet pipe. Blow the carburetor including the two tubes with compressed air if you have it. With the carburetor upright, blow into the fuel line connection. Air should blow freely. Invert the carburetor and blow again. There should be no air flow as the float needle valve should close. Apply anti-cease to the five carburetor bolts. Reinstall carburetor gasket onto the air filter housing. Reassembly is the reverse of these steps. Since this is a single speed carburetor, there are no adjustments. This worked for me. Replacement carburetor can be found at: http://www.amazon.com/Briggs-Stratton-498170-Carburetor/dp/B00176FTW0

After again verifying that mower drive belt is routed inside all belt guides and engaging all pulleys (see manual), make sure that Deck clutch rod and lever are assembled together with spring retainer in place. Start engine, engage mower. Deck blades should begin operating. If not, re-check all pulleys to turn freely with no slop. Also - with engine off - engage mower and check to see that belt is tensioned. If not, belt stretched or mower clutch mechanism faulty or needs lubrication. If needed, see link below for downloadable manual. Here is a link that might be useful: manual

If the battery is more than 3 years old--buy a new one! Then, clean up all of the connections, such as ends of battery cables, and especially the ground wire where it bolts to ground, i.e.tractor frame, or body of tractor. Use new bolts, nuts, and lock washers to connect the new battery! This will make sure you have a good electrical connection in the starting circuit! there is also, usually, a fuse in the line somewhere. It will be one of those new style flat fuses, and easy to find. See if it has blown. You'll be able to tell easily. The little curved piece of metal in it should look to be in one piece. If it has blown, there will be a gap in it, and a burnt look. Also, the bolted connections to the starter solenoid are known to get corroded and cut off current to the starter solenoid! And, any battery cables with a green powder coming out of the covering are ripe for replacement. The seat switch wires should be connected, to avoid hurting anybody who might manage to tamper with the mower if you aren't on it! Especially little kids!