2 stroke added to oil on mower help

Briggs and Stratton 675exi engine on a masport 800 series

Not mower above in title as they didn't offer masports.

We've had a team put 2 stroke oil in the oil hole on the mower above.

They've used it all week and now it's stopped starting and they have finally told me about it..

Never had the problem before and I have checked for compression and no damage in terms of cracks or splits in the mower engine block

Tried new spark and the spark is there, obviously completed emptied and refilled the mower with genuine oil.

Any other ideas would be amazing.