Engine smokes and has back pressure?

I have an ride on mower with a briggs& sratton 12.5hp syncro balanced i/c engine, model no 289707 type 0154 01
It was running fine but has now started to smoke and oil is getting into the air cleaner, if i remove the oil dipstick when running there is a lot of back pressure and oil spitting out.
The engine is now also very hard to start hot or cold.