Speed passion rotor
 

Where can I get a replacement rotor in the UK? Racing cars are out

I got a SP 5.5 motor from rcmart tried it for a lap and a bit then I thought I'd done my slipper:

Pulled in and had a check and its rotor/magnet release!! Pulled it out and the magnet slipped halfway down the shaft, I haven't seen anything like that before.

The one that was in there was a black one with red top that by the looks of things online should be for a 9.5-21.5

If this is the case then what is it doing in my brand new 5.5?? Little pissed if this is the case.

Whats the correct rotor to have or what would be a good for offroad?
Thanks