I have mounted a Brillelli 26SS on a CMP Hellcat. I would like to know if I can run this engine inside the cowl and not cut a hole for the velocity stack? In other words, can I leave the stack off and get enough air to the carb?

I would say that if the carb fits with at least 1/2" clearance then you would be fine running with out the velocity stack. The stack is there for when the intake to the carb is in free stream air.
You will get more of a response in the "Brillelli 26ssr" thread.
http://www.rcuniverse.com/forum/fb.asp?m=7311253

My experience has been that the velocity stack is more trouble than it's worth, especially if it's directly open to the airstream, outside the cowl. I tried one on my MX2, and found when turning right the stack got blanked, sucked in less air, engine would go rich. Removed the stack, ran the customary mod of fuel line from pressure diaphragm on carb back into fuselage interior, problems went away.