Certain things are allowed. BMF for one, is generally globally accepted on OOB because of what it brings to any model. Certain clubs might allow ignition wires or flocking, but that is probably the limit. No major surgery. No top chops or sectioning and the like, but things like smoothing badges, scripts, door handles etc. is usually okay.
I've been recently researching this because I'm building one myself at the moment. One major thing I just found out - if your kit is an old style front axle-through the engine block kit, there had better be an axle though the engine block. According to IPMS guys in my old club that is.