Incorrect hookup would kill it. Or motor wires coming loose while running. With the unusual terminal numbers I wouldn't be surprised if they got fried before you got them.
Diodes sound ok. The SCRs have to be removed from the board to do a proper test on them using the jig with the power supply and light bulb. Sounds like they are iso-tab types which is good, much easier to install than the non-isolated ones.
The ones that KB uses are iso too, I can get you some part numbers if you need to buy any.
Ciao,
Mark
ps make sure the board mounting brackets are not causing a short somewhere
pss the SCRs can fail shorted or open, usually shorted: bulb on (or off) all the time; gate having no effect.