The overall switch is generically nicknamed a "grenade pin" switch.
As Nigel says, if a standard plug will fit that is you best option. Or find a local engineering company that can turn a bit of metal to the correct dimensions, if a shorted plug will not do the job.
Or just by another alarm - you can find them for $5 / £5 or less on ebay and amazon etc., which is probably the cheapest option.
If you must use the original, look for a cheap one that has the same pin switch & discard the rest of the unit..
The same applies if you want a switch for another project - get a cheap alarm that uses one, from ebay or amazon.
Examples:
If you want a switch that closes when the pin is pulled, you can also use a jack socket that has internal break contacts; those are very common and cheap.