I have a 1976 Benelli 125 Enduro this has the same engine and electrics as yours. Because your bike does not have a battery you can not use an electronic system . My bike has been blighted with ignition problems since getting it back on the road. Although you have a spark the condenser may be the culprit ?. Try changing it to see of that helps. My bike would not start at first on further investigation i had stripped the woodruff key on the crankshaft putting the timing out. That said i had a 125 Turismo in 1981 and it was very fast,very light and reliable. I think the ignitions are now some 30 years plus old and need some tlc . I have just had the mag coil rewound at west country windings will let you know if that helps!