Not an expert, but this is what occurs to me at this point.
The 7 triacs is fine, many years of testing has shown them to be a good system. They cannot all switch at the exact time, but so close to it, I have not been able to see any spread on the scope to show that this is the cas or not..
If they fail. it is usually the one closest to the output track,..... rare, but that is the only one I have had fail on two ( of many) boards over a number of years. The triac you picture will be destroyed in a few pulses..... tried them. I even used 150A 1500v scr and triacs, but the seven parallel seemed more robust.
Any diac will do the same job really, and if a traic fails, it will fail short circuit, and the oscillator will just have a low freq squeal, and not rise, as all the power will direct into the transformer, not the cap.
So, what could be wrong.... check the primary windings connection/s............... are both bifilar windings connected, or has one been lost. This would account for losing half the output. It cannot be the triacs as I see it. They either run, or they are short....no in betweens.... ditto for the diac... never seen one blown so far.
Try another cap, in case there is a loss of capacitance..... else try the other transformer. The device is so simple, I can't find a complicated answer for you.
Big storage...... going to big low impedance tranny......... makes big spark.... simple really, so it must be either storage, or the transducer. The triacs will dump all the energy they can, only the small toroid and the two caps on the transformer primary can make any difference. The fact that it was viscous for the first few sparks, means the toroid seems to be not too restrictive...... so really means cap and tranny are the only ones left..... just can't find it in me to blame the triacs I'm afraid.... been wrong before too...
...................oztules