Nothing decent for the front which is a 19. most common is to get a crf80 or xr 80 front wheel which is 16onch and bolts right up. tires for flat track for my bikes are either bt45 b-stone which are good for dirt and parking lot training. heidenau also makes a 16 snow scooter tire which is also decent abd offers better dirt service but sucks if you go to a mini Moto or asphalt training. also, Mitas makes a 16 street tire that work well in both but not many shops or places sells Mitas.
SR241 Shinko "GoldenBoy" dual sport/trials worked pretty good. Way more dirt grip than the street tires using before....If "grip" is what you seek?
I really liked the k66 it felt more flat track ish, also used them for indoor mini moto and in rain on asphalt. They are little pricy, best bang for your buck are the 241's
They make a BT-45 for the 19 inch stock XR wheel. I have run mine on a small flat track and it worked fine.