Glenn

I guess this might be a silly idea but here goes for what it is worth 

Considering that a synchronous AC motor goes to alternator mode when driven above it synchronous speed it may be possible for you to use a motor in this way.

As I understand it the motor must be energised with AC at the appropriate frequency before speeding up to become a generator.

So, how about a cheapo inverter (maybe only 50watts or so) connected to an AC induction motor.  Of course the motor will likely not even start to turn but if powered by your engine and the revs increased to just above synchronous speed would you not have an AC alternator of much greater output than the inverter?

I dont know if this is a silly idea or not but if the petrol engine was available here I think I would give it a go.