I built a 1994 Toyota pickup and swapped in a OM617a mechanical diesel. It's a really fun party trick to unplug the battery and have it continue running.

In terms of security, it's my most secure vehicle. Mechanical diesel means its gonna need to be glowed which I have it setup as a push button and no thief will know this. As well, my shutoff switch is a toggle switch under the dash I leave to "off". It'll just crank and crank forever. And my biggest security feature? It's a manual transmission. Most see that and won't even try.

Security by obscurity