Hi, hoping someone can help me to establish whether I really have knackered my girlfriends expensive lawnmower !!

its a Briggs & Stratton Sprint 40, which I'm fairly sure is 4 stroke

There's all sorts of prelude to this, but suffice to say that the lawnmower is new-ish, was recently serviced and running fine til this incident.

I turned the mower on its left side, so that the spinning blade was visible, not noticing at the time that this was the side of the exhaust outlet. It ran normally for a while, but then billowed white smoke from the exhaust.

The gap between the exhaust and its guard may or may not have allowed sufficient room for escape of gases. I know for a moment i had a very small fire on the lawn there.

Also a reasonable quantity of engine oil had obviously been expelled from the exhaust. I believe that oil out of the exhaust is very serious indeed.

It sounded normal til it cut out and did restart once, the smoke beginning to die off til it cut again after about 30seconds and it will now not restart.

I fear that perhaps the engine was starved of oil; perhaps the pick up is on the side of the sump that gravity had taken the oil away from? Would I normally have heard knocking from the main bearings if that was the case though? And would it more likely have seized? (its not seized)

Or might the constriction to the exhaust have burnt out an exhaust valve?

I used to dabble in mechanics to cars, most ambitiously on one occasion completing a head gasket job (after much trial and error).
How simple are these engines?
What is it the most likely problem with it from the above?
Is it a foregone conclusion that the only realistic thing to do is bin the mower?

The spark plug is clean by the way, and the engine turns over and fires normally on a pull, without starting.

Advice much appreciated

Thank You