Is the MZ-12 PRO really a 12 channel transmitter? Graupner seems to double the number on their naming of everything, usually it would mean it's only a 6 channel. Their website does say 12 channel, I just don't understand why the naming isn't consistent.
You should be able to connect any of Graupner's 2.4 GHz receivers with that transmitter, but if you're looking for helicopter specific ones then you could look at the GR-18 AIR. This is a 9 channel receiver (but it will work with a 6 or 12 channel radio). The only benefit here would be the dual antenna set-up that can help with better range and greater connection reliability.
Hope that helped some.