Hi,
People who have these classic-type vehicles will very often
want to keep the working parts as original as reasonable,
sometimes at shows points are deducted if modifications are
such that are not 'in keeping' with the perception of the
vehicle. Better quality brushes and suppression would most
likely be acceptable, changing the unit for an alternator
would probably not be. There has been an issue about polarity,
a lot of earlier motor cars had positive ground (chassis),
but later it was thought by many that having a negative body
could actually reduce corrosion of the metalwork, most cars
are now produced with negative ground and have been for years.
The thing is, those who altered their early vehicles to this
standard at one time had points deducted at shows, but now i
think that alteration has become accepted, and now considered
ok as it does not alter any of the working parts of the car.
Yes, i realise the EMFs and the magnetic fields are altered
but they aren't physical parts.
A lot of the spikes could be coming from the field regulator,
which is an electro-magnetic device using contacts opening and
closing to give a 'Mark-to-Space' ratio on the field windings
thereby aiming at the 14 volt output from the dynamo.
If this unit is in its own case, it may be acceptable to use
another case of the same type to house an electronic version
of field regulator, the original could be kept with the cars
box of spares, so that if any purists should want to, it can
be replaced at some future time.
It might be possible to use the regulator from a modern
alternator to do this job, but the circuit would have to be
checked out, from what i recall the ignition lamp is involved
somewhere in this. I think that it is possible to make a direct
replacement with an electronic unit, such that the original
wiring is unchanged.
I suppose it depends on how far 'Belair' wants to preserve
the original appearance.
Changing the radio for a later type is unlikely to detract
from the originality, although many like to see the old car
'tube' radios, but the newer ones do sound much better.
Personally i feel that using a zener is probably the easiest
way, it could be placed anywhere tidily, but i think that 15
volts is a bit low, a weak battery could reach that on charge
with no load, i would suggest at least 17 volts, and make it
a large one, the sort of thing that motorcycles use.
Best of luck with it, John