lawn mower problems
Put the Champion J19LM or RJ19LM in. They are the same plug, except the R is for a resistor type. How long did you run the engine after changing the oil? Your hard starting is probably caused by the oil fouled plug (high oil level).
Also check your air filter. I usually just blow the debris from the air filter with compressed air. If the previous owner didn't know too much in respect to the oil level, consider changing the fuel filter and drain the gas.
See what happens with a new plug and the correct oil level before doing any further troubleshooting. Spark related failures are usually like just you turned it off (instantaneous), while fuel related generally run rough, sputter then stall.
What we've got here is... failure to communicate. Some men you just can't reach. So you get what we had here last week, which is the way he wants it... well, he gets it.