My guess would be that the keys are all alike. I know that all the keys for lawn tractors and school buses (within certain brands) are the same. Same with the cargo bays in the RVs.
You may want to google search for that key, or at least contact Cummings to see if they can send you in the right direction. I just replaced the key on my lawn tractor...had to replace the entire switch, as the key had broken off in the ignition. A spare set of keys were about $7.
You can check this site out, and either call or write them.
http://www.cumminsonan.com/contact/Good luck:)