Convert to 12 volts?

I have a 1971 ST50 which I am rebuilding, there are no electrics with the bike, would it be worth making it 12 volts?
Is it a simple matter of connecting two of the three wires from the alternator together? Is so which colours?
I have rewired bikes from scratch before and would be able to include a regulator/ rectifier, as far as I can see the bike originally only had a rectifier, a reg/ rec would give the battery more protection as I was thinking of using a small dry cell unit.
I would welcome any thoughts or comments, thanks.