Simplicity/ Kohler 23hp twin One cylinder sputters and backfires

Hi all and thanks in advance.
My engine is not running full power.
Both cylinders have spark. The right one runs fine alone. When i pull the wire the left cylinder runs but sputters and backfires.
My understanding is if the individual ignition modules (coils) spark that means they are fine meaning that they usually just stop working rather than output badly. Is that true? Or should i check voltage?
Also I noticed combustion( a little oily) coming from the breather and see that the grommet bellow is buggered
i pulled the valve cover on the bad side looking for a bent pushrod but the rockers where tight with no play so i figure the rods are Ok.
Does that mean a bad head gasket?
Should I take the cover off the cover they put around the engine to make repairs costly and look for signs of a bad gasket?
Or do i have to take it in for a leak down test?
Or any other ideas out there?
Just a side note At the same time this issue started, my battery is not keeping a charge or not charging. ill have to jump sometimes even if its been running a while and oddly some starts are fine coincidence? or part of the same thing?
thanks

Before you put your hand into your poskets, try the same test, but remove the kill wires before you pull the plug wires off.
There is a diode between the coils to stop them interfearing each other which breaks down.

Next is it a Courage or Command
Courages need to have valve lash ( 004") where as Commands mostly have hydraulic lifters so there is no valve lash.
Too little or too much can make an engine play up.

Thanks You think the diodes could keep it firing but badly?
And sorry how do i remove the kill wires sorry i dont know

Take the blower housing off.
The kill wires are the thin ones going to each coil and they simply pull off.
Because both of them are connected together they need a diode.
Search Hall Effect Trigger if you want to understand how they work & why they need a diode.
Some latter models have the diode built into the circuit inside the coil and these simply need to be replaced.
Check the valve lash first.
Always check the mechanical stuff first because that is easy, you can see it, feel it & touch it.