Four dry plugs resting on the cylinder heads and crank engine to check for spark.

It could be as basic as the dead plugs. most of us who have these bikes end up with a stash...

you can clean them in the hot flame of a torch, or your gar hob - wear gloves/protection -it's hot!

Update. It looks like plugs are not great. If I swap them around I can get different cylinders to fire.
All still spark when out of the bike.

New set on order.

It’s alive

Good news :)