Small 2 Stroke Self Charge PUBLIC

Voltage regulator input, for additional electronics, connected to the Battery.

Charge Voltage Regulator, consider peak current.
Component value calculator
http://www.electronics-lab.com/articles/LM317/

Half Wave Voltage Doubler
Voltage Doubler component values
http:///92jfpwv

Diode current ratings should be as high
as the Voltage Regulator current limits.

Capacitors, quit charging when full,
so are frequency dependent,
because of the amount of charge time they have.

One solution would be using
larger values for low frequencies and
smaller values for higher frequencies.
A diode in series with each output,
then connecting them in parallel.

A cheap (free) oscilloscope might help you
select component values.

http://www.electronic-engineering.ch/radiocontrol/circuits/osziconn/connect.html

Google
Computer Oscilloscope
or
Sound Card Oscilloscope Software

Sorry;
I may have been addressing a condition that may not exist.

A starter motor would normally be geared down, to develop sufficient torque to crank an engine over.
Upon concluding cranking the engine, the starter motor mechanically disengages from the engine.

If a starter motor were designed to stay engaged, it would work as a generator, and any manufacture would use this.
A DC starter motor, would also be a DC generator, not AC.

I suspect the AC output you have, is the primary Flyback.
Manufactures already use this boost power to charge the battery, with the battery powering the horn and lights.

If this is the case, voltage doubler and regulator are not necessary for charging. Just your USB regulator, to the battery.

Adding more power at low speeds.

Low frequency;
Disconnect C1f. Connect C1s. Increase the capacity of C1s,
until just before the output voltage starts to drop.
(Slightly under capacity.)

Higher frequency;
Disconnect C1s. Connect C1f. Increase the capacity of C1f,
until the output voltage starts to drop.

Connect C1s and C1f.

The high frequency (fast)
voltage should output over the full range.
The low frequency (slow)
will add power at the low frequency end.

If more power is needed at additional frequencies, add more circuits.