Semis are much more common, the rebuilders do those a lot and that's the way to go for them. On my 72 ECM most rebuilders didn't want to touch it, and the ones that would take it said "we'll give it a try". You might have better luck on a 75, though. The 72 engine was an old PT pump (they call it a centry pump system) driven by a very basic ECM. These were used in a few ag and marine applications. A rebuild was going to be around $1500, brand new was about $4,000 - but the rebuild may not work, of course. They change some components and hope it works, and the warranty is only as good as the place doing the work. Again, in my case it wasn't even the ECM that was bad, so if you could get Cummins to agree to put a new one on and give you a refund if it doesn't work that'd be the way to go IMHO. I doubt they would do that for you, though.