Have you had your stator coils checked by a ohms meter ... this would be my first check.
STARTING PROCEDURE ... kick over 3 times no throttle with decomp, if no decomp take it thru full stroke x 3
Bring piston up to TDC on comp stroke and give it a good kick with no throttle and
choke on, let it warm up a bit or it will coff and die on you.
Have you got the valve timing right
?? if no check and set - take off mag cover you will see a witness mark ( V) on the casing just below mounting bolt front upper and marks on fly wheel ( T ) this is TDC .... cam line should be inline with top of case pointing to back of motor.
Edit ... this will also see if cam chain has reached it's limits.
( exhaust and inlet ) Valves should both be closed and not rocking .
To set valve clearance undo the back nut and turn the lead screw slowly until it bottoms out ( SFA resistance ) = zero play ... now back it off 1/4 of a turn and tighten up the back nut ....... got all that
Spark .... you should get a good spark by kicking it over by hand that will throw about 4mm from the coil wire ( cap removed )
Check just above engine number were wire's pass thru casing that they are not cracked ( known to )
Are the pick up sensors faulty ( just above top stator )
Have you got a good earth @ coil mount.
Alex these OLD girls are bullet proof, and once sorted they can give you many years of Fun
It's a matter of going thru all the above one by one not in multiples.
Cheers Stewart