Sovereign mower oil problem (Briggs 3.5hp)?

For the first time this year I moved the mower from garage to a lawn area I cut. This involved lifting it over a steep step in the garage and transporting it in the back of my car. Next day when I was going to mow the lawn I noticed there had been oil leakage on underneath. There was also a spilt pin on the ground(although that might have nothing to do with it). There was also oil on the red painted top of mower, underneath the engine. Tried to start it, and it did start but there was white smoke coming off the exhaust which also seemed to have oil on it too. Given there was no oil on the floor when I removed the mower out of the garage initially I suspect I must have tipped it at such an angle as to let oil leak from somewhere. Apart from refilling the lost oil, what should I do to get it back to normal ie not smoking etc?