You can run the 5800 rotor and endbell on the 5.5 motor (stator). I could not fit the new rotor (has carbon or kevlar thread wrap on it to prevent explosion presumably) into the old stator (old 5800 can, may have just been the circular phase connector plate, not necessarily the actual stator being too small). Using propellors, I saw minimal, if any, power difference with a rotor swap.
If it's too much power for you Dave, try this:
http://www.mabuchi-motor.co.jp/cgi-b...AT_ID=rs_380ph
Also if you are paying attention, it looks like the ROAR rules have changed to allow increased variation of stack and rotor sizes in brushless.