Just a bit of warning about changing the plastic turn knob, I tried a petcock from MC16 tank that I have on the MC21 I am currently rebuilding. It fitted ok, but the MC16 plastic stem was too short so i just thought I could switch out the plastic turn knob. Wasnt able to as the shaped bit that sockets into the plastic knob was a different size between the 16 and the 21 so in the end i had to get a 21 petcock.

cheers