Forget it,you need a laminated iron 'core" of high silicon transformer steel that maintains VERY close proximity (less than 1/32 " or a fraction of a millimeter) to the circular flywheel,to close the magnetic circuit and envelop enough magnetic flux.Otherwise the induced voltage will be too low,that is to say it would then take many,many turns of (of necessity) thin copper wire to get an adequate output,say 6V ac or so.But this coil would have a high resistance,probably into ten's of ohms ,with 6V and 10 Ohm source resistance the max output is about 0.6 Watt dc (including rectifier diode drops) that is 0.1 amp at 6V not much.Unless you have a precisely shaped coil in very close proximity and enough space to put a winding on it ,you'll be spinning your wheels.I'd say forget it, unless you absolutely need something to occupy yourself with (Think Sisyphus and his rock).If you have an HT engine you'd be better of fashioning a battery charge circuit using thr White Wire,you can find a few I came up with in a thread started by Etacovda.titled 'Solid HT white wire data', in which I posted some schematics to charge 6V or 12 V batteries.Good for about 0.5 Amp charge current for either a 6 or 12 V battery.