A1 petter engine

Without much doubt it is the magneto, try cleaning the points with a piece of very fine wet and dry paper, it sometimes works miracles if not an expensive mag rebuild will be called for or a replacement mag. ( I have a mag I would sell ) John Southall.

Clean the points with a bit of not less than 400 grit W&D, soak a bit of thin card in meths and repeatedly draw it through the points afterwards. Stop when it comes out clean & hold the points open for about a minute to let the meths dry off. DON'T use petrol for this as it leaves an oily residue.
Check the spark at the end of the lead, not the plug.
Still no luck?
Are you sure the mag is rotating? Yes? Might start getting expensive now then ;o((
Come back and ask for further advice if you need to and I've sent you my mag crib sheet separately.
Regards,
Kim Siddorn
I'd rather have a free bottle in front of me than a free frontal lobotomy!

Basically mate cheeck the magneto, strip it down and clean it there pretty simple just watch out for all the bits, PS dont not leave the coil out of it for every long otherwise it loses its magnetic fields.
nemore help required or more detail wanted.
MAIL ME, im more than happy to help.