swapping a louder siren into circuit?
It sounds like a cheap enough experiment. The only thing to watch is the current through the reed switch. If the larger siren draws a much higher current the switch contacts might burn out.

If you have a multimeter you can take some measurements.


Switch your multimeter to the highest range on the mA mode, plug the leads into the right sockets and wire the meter in series with the load or across the reed switch contacts (which is in series with the load). If the current is small you can select the most sensitive range that doesn't give an ÒL`(OverLoad) indication. Note the reading.
Connect your new buzzer to the battery or power supply and test it. If all is well then break the circuit at one point and connect the meter in series with the load. Note the reading.
Switch the meter back to 'V' (volts) measurement and, if required, move the probe leads to the correct sockets. This step reduces the likelihood that you will forget the next time you try to measure volts only to realise too late that you have shorted out the supply with your ammeter.


Then decide whether to go for it or not!