Now that I see the rest of the schematic, the MR56 is actually a 5.6V zener, 1W and setts the bias for the two 15V in series zeners.
For the 5.6V zener 1W would be fine, 10% tolerance.
The 15V zeners should also be at least 5% precise voltage tolerance and they may have to be closely matched in voltage at 150mA of test current from a collection of them. Since the 15V zeners are also part of a crowbar protection circuit, this is another reason the 15V voltages here might need to be precisely set. The 15V zeners may have to be 5 Watt zeners mounted on a heatsink. Be advised, the 82 ohm at the collectors of the 2N3053's, needs to handle up 3 watts of power under overload conditions,and the pair of 39 ohms at the emitters (at least 1W) and the 2N3053s will require a heatsink.