Hi Robbie,
A good mate of mine as recently bought a T1R and he has replaced the centre brace with a graphite (I don't know which one but it is a single strip of carbon fiber) and his car's rigidity is at least a match of my EVO2.
In short I don't think you are going to notice a significant change in your handling with the plastic chassis.
The only bits that may not work are the battery straps otherwise I can't see why you will need to change any other bits.
I hope this helps cheers
Mark