Hi David.
D1, D2, D3 are just used in a multiplier. They must withstand the total output voltage of the primary side.
I'd be looking to use 1500V diodes. They don't need to be anything remarkable, they're only working at mains frequency.
I doubt the resistors are 10R across the diodes. In a high-voltage circuit, they'd self-destruct in no time.
Can you check again? I'd expect something in the kilohms range, if not higher.
The 5M6 resistors are just forming a slow charging path for C5, to make a crude timer to trigger the SCR. Yes, I expect them to be in series. They won't be dissipating much power. Around 20uA, so about 20 milliwatts....
I think 200R looks reasonable for that snubber.
The SCR will dump a decent whack into the transformer. It's going to need to be at least 1KV rated I think too, and a bunch of amps. Your SCR claims to be 1200V, which might be enough. Current will depend a lot on layout and the transformer.
D4 will be to quench any backEMF or ringing from the transformer firing. It probably wants to be at least 1200V or more.
The 220R series resistor should limit current to about 4.5A, so a 10A diode will probably have enough headroom.
I'd be looking to oztules to support or debunk my thoughts, however