DD,
Don't run your house in the US at 50hz. It can cause you a lot of trouble. Most small AC - DC power supplies can handle 50hz, but your appliances or HVAC won't like it. I would take Listeroil's advice and get another pulley to run your alternator up to 1800RPM.
Regarding the 120 - 240v issue, you could install a 240V - 120V/240V transformer or a 240V - 120V transformer (not center-tapped) to derive 120V. Your best bet IMHO(if you want to use the existing generator head) is to use a transformer with a center-tapped secondary to derive the 120 legs. Ground the neutral (center tap) at the transformer or the subpanel, but not at both. As with the generator, make sure you don't back feed the transformer from the utility. If you could get a small transformer with taps, you could also adjust the secondary voltage if needed, usually up to 5% in 2.5% increments.
Also, be aware that the generator head may be grounded to the chassis on one leg.
Hope this helps,
Sparkie