Petrol Mower won't start?

The last time I used my mower it started to slow down and then stopped completely. There was no smoke, but oil came out from somewhere and filled to area on top of the mower. I had not topped up the oil so it was not overfull. My husband took the engine apart and ordered and fitted new gaskets, cleaned the engine well but it still will not start. When he pulls the start cord it does not really kick in enough and there is a loud knocking noise.

Its a Honda petrol mower.

Any ideas? Thanks very much