You could just replace the diodes with BAT41 diodes. Similar voltage rating and similar capacitance, and probably will work well enough as a drop in replacement. Leaving the heater wiring open on pin 3 should do no harm. Will enable you to do 100V peak, and more if you place 5 or so in series, but probably I would not probe more than 200V with it.
Don't know how they get a 1200V rating for AC with that probe, as the valve is only rated for 300v max. It might be 1200V DC across the coupling capacitor, with 100 VAC peak only.