Progression of regs...

9.1.4.A, "Vehicle modifications are limited to those listed herein. Unless a particular
modification or part is approved in these rules, the vehicle and all of its
relevant parts and assemblies shall be stock for the correct make and
model of car."

In other words, unless it says you can, you cannot.

However, 9.1.4.G.e, "It is permitted to use the OEM intake and throttle body from
either the chassis or the installed engine."

Otherwise....stock required.