it depends on how much time and money you've got...
with a budget that most of us would only dream of, you can get your red line as high as you want it.
basically you have to build your valve train to handle the higher rpms.
from there, you would have to balance the rotating mass.
the better the balance the higher the RPM. to get some crazy high number, you would need to increase the oiling ability.
the shop that you had balance it, would have to knife endge and polish everything. "custom" bearings so the tolerances are tight.
the higher the redline, the more its gonna cost
how high to you want it?