As fast as you want. Again this depends on your gearing, if the track is hard packed, long strights, long jumps with smooth landings, go with a smaller rear, larger front gears, the faster you will go. For deep, thick, heavy, or muddy tracks, you will want a larger rear, and smaller front gears for the torque.
fast enough
about 40mph
210
about 60mph
it all dipends on the bike
A 70cc dirt bike will achieve a top speed of around 40mph
not very fast
55 to 60 its fast
how fast does 250cc Orion dirt bike go
1 mph
59 hp
16.557 fps