Hello Mike,
The HT leads are typically a source of trouble if aged; you say you replaced the coils... can we assume this includes the HT leads... and even better the caps?
Other areas to check is the set-up correct... points gapped @ 12-16 thou", plugs (NGK D8ES) at 24-28 tho"
Is the points back plate clean / well earthed? Are all the leads / connectors clean & in good fettle?
In answer to your alternator question, the battery voltage will nominally be around 12.4V, and rise to abouts 14.5 by 4000 RPM. I guess this 20% increase in low circuit voltage will directly increase the HT voltage by 20%.... but whether this means a 20% better spark I'll leave to smarter folk than me to say...
What will be very relevant is the starter motor cranking will drop the voltage significantly... is this your problem perhaps? Off the top of my head I reckon it'd drop to 9 v or so; and even less on a bad battery or with worn starter motor.
(a trick used by manufacturers to overcome this was to fit ballast coils... which typically ran at say 6v via the resistor. Except when you cranked the starter.... the ballast resister was removed from the circuit allowing 9v or so to the coils... and bingo a big spark for starting. Once running / starter motor off ... the resistor switched back in & coils now back at 6v)
I digress...
An option to consider might be fitting an electronic ignition to boost your spark... definitely pros & cons to these... a few threads around this website if you look.
I've a breakerless electronic ignition on my '750.... I can't tell you it particularly runs better. But, it certainly starts more easily... via the starter motor regardless of months standing.
Simon