As long as you can get it started you can fly it. Which sounds dodgy but actually it's a fairly self regulating system - no flight school will rent to you without a thorough check out and no insurance company will insure you without time on type or dual received. Bizarrely the only light helicopters requiring specific training in the US are the R22 and R44 by virtue of SFAR73 - implemented years ago to slow down Franks killing spree. They require specific training and experience and their own flight review. But a flight review in an R22 means you're good to go in an MD500 as well (Or an airplane for that matter, but that's another story..)