There is a typo in the original schematic, and Tony van Roon not only didn't catch it, he compounded it by adding one of his own. Those diodes are Schottkys, not 400 volt zeners (why in God's name would you need a 400v zener in a solar-powered battery charger ?). The closest number I could find was 1N5717, but I didn't find a datasheet. I suspect there are any number of Schottky rectifiers that will work. Pick one that can handle an amp continuous and will stand off 20 volts reverse, and it probably will work. The zener will probably also work, but not as a zener - it will function as an ordinary diode. Schottkys are more efficient - lower voltage drop and faster reverse recovery.
Tony said the diode was good for 7 amps continuous at 400 volts. This is 2.8kw! The actual rating is 7ma max, but this is reverse current