Evening.
Some points to look at
Ref the spark plug, A couple of raps with PTFE tape round the plug thread will seal any thread damage, If that doesn’t work next paint the cylinder head sealing face with something like pot black and put the plug back in whilst still wet.
Leave to dry and try again. If it still leaks you need to check for a cracked head.
The lower part of the crank case. Check the drain plug fibre washer, if over tightened they tend to crack and leak.
Other than that you will need to identify exactly where the leak is coming from.
Primary drive chain, is that oil mist Lubricated from the valve chest? Maybe just finding its way out and on to the lower crankcase.
Additives, don’t, they will cause problems to the crankshaft seal, magneto seals and Dynamo seals.
Are you using at least a straight 40 oil?
Hope some of that may help
Steve