Just to let you know what we are up against over here, I thought I would post the generally accepted rules of MT racing over here in the UK.
Small Block Class
Types of truck included in this class~ TMAXX, TMAXX 2.5, TNX, REVO
No big block .15 engines
No medium block .21 engines
No 4wd stadium style trucks (this includes CEN type trucks)
Bump start ok
Any .12-.18 small block ok
Any suspension mods ok
Any exhaust system ok
Must utilise MAXX size wheels and tyres min diameter 5.1" min width 3.1"

The Unlimited class has different rules obviously but the ones that relate most directly to CEN are these-
No small block engines allowed
No 4wd stadium style trucks (this includes CEN type trucks)

So there you go, pretty cut and dried. These rules were taken from the Maxbashing forum, but it certainly seems that most UK clubs use a similar set of guidelines and the BRCA has the same rules with regards to the wheels.