sounds like an interesting concoction you've got there!
this years uk-microx race (which is free to enter) is running to 2010-2011 micro section rules,
http://www.uk-microrc.co.uk/ukmicrox2010/rules. so that will give you an idea of what is legal in which class.
In order to split the classes running of 2wd in 4wd and vice versa is not allowed at sanctioned events, but no club i know of will stop you running on a club night.
if you can make it 4wd drive again, that would give you entry to all of the nationals, if you want to of course!