demobilizer "key" lost?

I have an older F150 with an older demobilizer and my daughter lost the unit she needs to deactivate it. it's an older style looks sort of like a cross between a spade and a cloverleaf. We've been told that in order to bypass it the whole truck will need to be rewired or we need to pull the whole unit and have a new one installed. Any suggestions and what would the cost of a bypass realistically be?