Hi, I think the situation in the USA is you don't actually need a licence to fly an ultralight (I know several people who just fly them from thier back yards). The trick will be finding someone who will rent a machine to you. Most of the ones I found were more for training or tourist flights (brilliant floats quiksilver in Key West).
I also have a UK microlight rating and when I wnet to the states, I ended up just flying Cessnas (as it was actually cheaper to use my PPL). If you do a google search for ultralights you should get a fair few hits. Florida has some - but few and far between.
Good luck
fluffy