I don't have a recent experience with CJM. But I had them make the prototype rotating retracts for me for the H9 P40 when it came out several years ago. They worked great and these guys are great to deal with. I sent the stock retracts to them and they made pneumatics off that foot print. Don't have to send in the wings if you know the dimensions of the struts and trunions you need. The P51 is much easier to make than rotating retracts for P40. Robart does a good job, but they take a long time to produce. I just had a custom set built for me for Ziroli warbird and it took Robart 2 mos to make, rather than 2 weeks they originally promised to me.
By the way, I don't see why you cannot find retracts from Robart that will easily fit your P51. It's not a big plane to find struts and trunions for.