I have a Murray riding lawn mower that I can't get started, I have changed the key switch and had the starter checked an it is okay, and the battery is good. It has a briggs and stratton 18hp motor, the motor cranks without spark plug but won't crank with spark plug, starter draws 20 to 30 amps, is that normal?

Can you give us the actually engione model number for the B&S? Have you checked your oil level to make sure its just right and not overfilled? Done a compression test yet?

If there is oil in the combustion chamber, that might do it. My father in law had that problem with his roto tiller, thought he was helping preserve the engine by dumping oil in it in the fall:) 20 to 30 amps doesn't sound like very much oomph. Sure the battery is good? Turn your lights on while cranking, if they go dim, get the battery checked again. Batteries can be deceptive creatures.

Make sure engine has a good ground to battery , this will cause it to pull lots of current.
Luck