Many years ago when I use to fly a volkswagen powered Gyrocopter the norm was not to use a altenator as it was too heavy.
As the power requirement was only to run the ignition only a few amps was required.
A Davis Craig themofan motor will put out up to 4amps and is small.
The bearings have no trouble in handling the loads , but the motor must be spun hard......about 6000rpm.
The drive was usually a large O ring or sewing machine O type belt on a home homemade alloy pulley fitted to the end of the thermofan motor.
A 10 dollar bridge rectifier from dicksmiths was used to ac/dc and stop battery fed back.
I think the out put is dc,but the rectifier module was a handy way of bolting up a diode.
If you want to get fancy a rectifier from a Jap motor bike will ac to dc and have built in current control and the motor bike wreckers sell them chip as chips