If an OHV 17.5 it would a 310000 series.
If several good possibilities. But first changing the plug with a known good plug; new don't always mean good. Plug can fire quite well out of the engine and fail under compression load.
You may have an ACR problem too since you have been having to boost the battery to start or you simply have trash in the carburetor's fuel bowl blocking the main jet as it is sucked up as the engine starts.
A couple notes here on these engines. They prone to both the ACR on the camshaft to breaking and for the head gasket blowing between the cylinder and the push rod galley. The first will cause very hard to turn over condition as the engine is at full compression and starter simply don't have enough power due to voltage drop. Most operators simply think it is a bad battery, a charging problem or bad starter. The later will cause engine to use excessive oil without even smoking resulting a condition where you have no oil and breaking the piston connecting rod.
I don't think you got a broken rod but may still a blown head gasket as it will hit. If you can do a leak down test to verify that all is well in that respect as preventative maintenance check.
