Don’t go daft and it’ll be fineI wouldn't use easy start, as it's far to harsh for something you start by hand.
I personally use a bottle of LPG with a torch, but have in the past used a small gas bottle and plumbing torch.
Question: Does the magneto have an impulse start ring? If so, and it’s not working, that may mean the timing is set a long way off.
If it does, the timing needs to a smidgen after top dead centre at cranking speeds. I have one engine with each cylinder almost two litres - it has an impulse start on the mag and you don't want to be hand-cranking that if it fires before top dead centre!
You still need to check your timing. The magneto could have slipped on the shaft or something.

Stuck valve or wrongly adjusted tappets, (if applicable!)
I took the head (?) off which is just cooking fins and the plug. Valves are in the block, the valves are pushed from within the block, not sure if it’s directly off the cams or if there’s push rods but anyway. They seemed fine, or fine enough for something 70 years old! I applied a thin bead of sealant to the head gasket, a 1mm sheet of copper.
I wouldn't use a sealant, just anneal the copper gasket
There may be a cover on the side of the block near the valves which should expose something like this on one of my stationary engines
View attachment 298924
Have you done a compression test? That would rule out carb or ignition problems if it is low.
I've had spark plugs that worked fine one day then didn't the next! It could be that simple? Sorry if you've already tried this.
it may be worth joining a couple of the stationary engine and old tractor groups as there is bound to be someone with a few plugs willing to help.