Hey maybe I need to explain further. The engine is on a dinghy. The VERY last thing I want to do is add more complicated electrickery AND a battery to simple dink.
I want to have the engine with it/s little white light as a self contained unit. I have done this before with a Johnson and just ran the 12 volt bulb straight off the lighting circuit with no rectifier or regulator.
The output from the charging coils is ALTERNATING CURRENT.
On open circuit and nr max rpm we get about 100 volts.
So what resistor bulb combination would work.
Would an LED work ? I did not think so as the max reverse voltage is about 5 volts and positive going is not that much higher?
The bottom line is that the voltage at the bulb when operating must be less than the voltage it's rated for. There is no trick. While you say it may reach 100V I assume that this droops significantly under load (but now much load I don't know). If you really want to do this, I still think the easiest way is just try high voltage bulbs. An incandescent bulb is resistive so adding a resistor doesn't add much value. Unless you just don't have 24V or 48V bulbs on hand. The other variable is power. Assuming that the output voltage droops under load a larger load will cause it to drop lower. So a higher wattage 12V bulb may pull the voltage down to 12V and work fine.
I take back part of what I said about LED's. With 100V AC I don't think they are likely to work but again it all comes down to whether the voltage falls to an acceptable range for the particular module.
But again, this is all guesswork without knowing the parameters of the output (voltage, frequency, impiedence (how much it drops under load)).
A cap creates an RC filter which will lower the voltage in an AC circuit. But a resistor also lowers the voltage. The RC filter would also depend on frequency of the AC power with higher frequency equating to lower voltage. This is somewhat interesting because it might help cancel out the effect of voltage increasing at higher RPM's assuming the frequency is tied to that.
What your probably thinking of is a zener diode (or transorb) and a resistor. Zeners can clamp to a particular voltage by dumping all excess current (kind of like that emergency drain at the top of your sink). You'd need back to back Zeners in this case due to AC and a resistor would help handle excess power but specifying them correctly would mean knowing more about the output.
Anyway, this is way more detail than needed but it's the result of just not knowing what that output is. When I suggested battery before I meant AA's or D's or whatever. A handheld battery module with an LED can run for hours. What about a flashlight? Still a nice easy option.