Even if this was possible, you wouldnt want to do it, you would totally loose velocity and it probably wouldnt produce power below 13,000rpm
A stock A12 engine would probably make more power
Go have a read of the Sticky Thread by Matt Hutchens about building a RB26, he talks about porting the RB26 head, I guess this would loosely apply to a CA18DE as well. He says DO NOT increase the size of the port, just clean it up etc and remove any lips and imperfections from the casting and original machining. Its interesting.