There was an electric starter option offered, but it came with the bike as an integral part, where the starter motor bolted on to the engine case.
There was a bolt on option offerered by NField Gear for a time, but I doubt many were ever sold. I don't know how they secured the starter motor.
I think that your best bet would be to either bump start the bike like a racer, or sell it and buy another one which has the electric starter already built on to it from the factory.
Be aware that the electric starter systems on these bikes are some of the most troublesome parts ever put on a Bullet.