I'd love to see where you get with this and if you can get the key to work for engine ignition. Getting the immobilizer to work should not be too big of an issue, it's been done before a few times. I've been gathering parts for a while to do some other engine mods and from what i have been able to gather from testing wires and from other MB cars is that our cars (and r170 chassis cars) use the switchblade key. As you know and when you turn the key to start the engine it sends a non CAN signal to the ecu. The m113 cars don't do this, they have the newer MB keys and send this signal over CAN.

What i would probably try to do is get one of the key assemblies from the newer MB cars and find a way to mod it so that signals from the existing key assembly will trigger the CAN functions on the newer assembly (a sort of piggyback setup). You could also upgrade the crossfire to the newer assembly but then your probably also looking at replacing security modules among other things.

I haven't gone far enough to actually capture CAN signals, but i would imagine replicating them with custom modules would be very difficult considering the amount of non essential data that would most likely also need to be replicated. Considering how much VIN and vehicle data gets sent to modules that don't really need it, i would imagine the key assembly also works the same way. CAN is essentially low level networking, the traffic may even be encrypted.

Anyways i'd love to see where you get with this and maybe you can think of something that i haven't.