Most research into Hungarian Gypsy folk music (rather than the music of the Gypsy professional musicians) postdates Bartok's time - he couldn't do everything. If you can read Hungarian there's quite a lot of information about it available now, and it's increasingly being rediscovered by the folk revival crowd.

A pretty good book on the range of music played by Gypsies in Hungary is Bálint Sárosi, Gypsy Music, Corvina Press, English translation 1978. There are also a few recordings available, like the Fonó CD Csenyéti Cigányok (2002), documenting the music of a single small Hungarian Gypsy village.