New cranks are as scarce as hens teeth. After an unnamed service turned the crank cheeks and made it unsuitable for a belt drive I went looking for a new or serviceable crank. None of the major parts suppliers had any. I could have purchased a billet crank, but didn't need the strength and didn't want to spend $2K; I kept looking. I found one, in the original Berliner box, that had been sold to a dealer in Pennsylvania, was resold twice and ended up in New Orleans. I bought it for $550 delivered.
I found several alternatives, to use the word extremely loosely; the majority of these looked like they are done duty as anchors, all were rusty, part number references were equally loose, hard to say what they came out of, and the prices were totally outrageous (Ebay--where else?), the least expensive of which in the $700 range.
I should have started calling second tier vendors, but I wanted it NOW. If you have the time to be patient, make the calls and ask the right questions you will find a serviceable crank. Beware of '72 and '73 750 cranks as they have a reputation for not having much of a radius where the pins are joined to the cheeks. A radius can be machined, but get the pieces magnafluxed to insure that cracking hasn't already started; I suspect that the cracking may be due to the factory use, initially, of non C3 spec main bearings; others will have greater knowledge of this.
My first and last Combat crank had these cracks and I was dumb enough to think that I needed a bullet crank, 'cause, of course, I was planning on at least 500 RWHP at 10K RPM...
There are enough builders that install billet cranks for power seekers that they should have a good selection of cast-offs (no pun intended).
There are plenty of members that, hopefully, chime in and help identify such builders. Think positive thoughts, want you want is out there.
Best.