I've built one of these amongst other things to raise money for charity, with a switch to select a buzzer or a really loud siren (the buzzer so as not to give older players a heart attack).
Anyway I havent tried this and it depends if your using the thing inside near mains wiring or not, but if you replaced R2 for 220k and the 555 for a 7555 the timer trigger input will probably be sensitive enough to pick up someone touching either electrode, you might need to experiment with the value, too high and phones/someone walking past might trigger it.
(the circuit shown doesnt really need the thyristor, rewiring the 4017 would give the same function)
Another way would be to use a capacitive sensor and/or a prgrammable device but thats getting silly for a simple machine.