Just make sure that the polarity of your connections is correct. Guys crank six volt starters with 12 volts all the time. Truth be know you could probably coast it down a slight grade and get up enough speed to clutch start it and once the generator spins for a few seconds it will put out enough power to fire the engine up. That is the one and main advantage a generator has over an alternator.
Cherrs mate, in my rush to get this going having waited nearly 6 months for it to arrive I forgot the previous owner had converted it to 12v.............Its on the charger now and hopefully start it tomorrow