The problem is that standard ignition inhibitors are usually easy to bypass.
Are you intending to ground the ignition coils (points/ignitor connection)?
Otherwise there are lots of code solutions.
I use to use SCRs (~transistors) hard-wired to keys or decoder that had to be sequenced right for final connection. Problem is - rip out the unit and simply short/connect the output wire... That could be overcome by running keypad wires to remote SCRs etc etc.
But it sounds like you have poor security if you think people can enter your vehicle - whether they want to steel things or start it...???
What alarm system is fitted?