The only way to diagnose the fault is to approach it methodically. Start with the plug (try another that you know is good), check the HT lead for damage and that it’s connected to the coil (it screws into a bakelite adapter which has a carbon brush on the other side which makes contact with the coil).
Once you’ve done that you can check that the points haven’t closed up or got some debris between them. Condensers can be difficult to diagnose and will give a spark when cold which fades out as it gets warm- try another condenser that you know is good. The same can be said of the coil- some coils will spark when cold but fade out as they warm up- and the only way to check this is to replace with a known good coil.
Does your spark come back when the engine cools?
The only other thing that it could be is the start/stop switch; disconnect this to rule it out, it should connect to a spade connector on the rear of the stator plate (the thing that the points/condenser/coil are screwed to).
I hope that helps. Above all, don’t try to take any short cuts; a methodical approach will always find the problem but a short cut can lead you down many a false alley!

The usual one is the timing has slipped again…..

If so, remove the flywheel completely, and clean the internal taper of the flywheel and the external taper of the crankshaft, ideally with brake or carburretor cleaner, but clean petrol will do. Also ensure that the tapers are undamaged. Slight burrs can be dressed out with a fine file or emery stone.

Replace the flywheel and time the engine correctly do not apply any lubricant of any sort to the taper. Then, put the spanner back on the flywheel nut, and hit it with a hammer to tighten it up fully.

Villiers originally supplied a “Hammer Tight” spanner especially for this purpose.