Are there even "real" (functional and safe) ultralight helicopters? I've been around the ultralight community for a number of years and can't say I have ever actually seen one that was a functional helicopter and classifiable as an ultralight. I've encountered two attempts in historical reviews related to my professional work but given that both of them wound up with a dead pilot on the first attempted flight by the builder/"pilot". That said, neither of them were registered aircraft but I'm not sure if they were technically "ultralights" by definition either.
There are those spindly little things advertised in Popular Science and other similar magazines from time to time but I wouldn't try flying one. A now deceased friend of mine (Grover Krantz, physical anthropologist and now a museum display) built one a number of years back but his wife forbade him from flying it. According to Grover when I asked him about it over lunch one day, her comment was "You try to fly that and it doesn't kill you, I will."
From a physics standpoint, the weight alone for the engine necessary to provide a safe amount of lift for an average adult plus an airframe would probably put it out of the weight limits for the ultralight category. This is just a rough guess though but I don't think a two-stroke would cut it.
The other thing to remember is that flying a helicopter is not like flying an airplane. It's not something you want to try without training (not that trying to fly a fixed wing aircraft without training is smart either). They are inherently unstable and brutally unforgiving of inattention. The kicker here is that training for helicopter flying isn't cheap: it makes fixed wing training look like a steal. Just some things to keep in mind.