http://www.rail4you.ch

http://www.trainsahead.com/c_sine.htm

Or you could try fitting the later Fleischmann armature:

Part number of the main part is 504603 called "armature" in english, i.e. Rotor with flywheel-mass

The set is no. 504604, containing the rotor (armature), 3 pinions (10, 12, 14 mm) and 3 washers.

I do have an alternative exchange rotor with part number 504606 - cannot tell whether there is a difference to 504606, electric resistance seems the same (13 to 14 Ohm) only that some flywheels do have a green dot, others a black dot or none at all on the metal bit.