Poulan Riding Lawn Mower Won't Start?

My Poulan (18Hp Briggs) riding lawn mower won't start. I changed the oil and filter, however I did not fill up the oil to required amount, as mower was on a slight incline. I added 1/2 quart and it started very well. Put into gear and started to move mower to level ground.

Then the sound came! Like I dropped a tool into the engine. I stopped the mower and checked the dip stick. There was no oil level and white smoke. I forgot that filter was dry and that any oil put into engine would go directly to filter. There is no real resistance when I turn flywheel - after each revolution there's a clunking sound. What have I done :( Can I salvage it? What are my next steps?

By Steve O.