I think I'd go with a true ribbon if possible, they're a bit better than the Neo 3, not that the Neo 3 is bad. Maybe look at the Heil drivers too. I'm not really up on what's best -- you can look at Parts Express and do some research. (Actually, the Raals are best but they're very expensive.)
Also, don't forget to check the sensitivity of your speaker and make sure the ribbon isn't less sensitive. Try to get a tweeter with the same impedance as the existing driver, which is probably fairly close to the nominal impedance of the whole speaker -- but look up the driver specs, if you can. You want to match impedance and sensitivity so that the crossover works correctly. (There are ways to correct for this but you'll need to build a new crossover, and needless to say that's a bit of work.)