any small SCR would do , even in a TO92 package.I resimulated the circuit with 1k for the buzzer instead of 100K. It works very nicely. Now to dig up an SCR.
I choose the 1uF and 100K values because with the 100K resistor substituted for the buzzer the circuit wasn't working. I didn't know that a) the circuit was bench tested, and b) the required resistance to substitute for the sonalert; which turns out was too high to make the circuit work. In short I butchered your bench testing because my sonalert-resistor substitution was too high in the simulation.