For a shop installation, some use refrigerated air dryers. A cheapo is available from HF https://www.harborfreight.com/compressed-air-dryer-40211.html Still, $400 and only worth it if you do a lot of blasting and painting. Garage Journal has a number of compressor/ dryer threads with some decent homebrew solutions.
Those are Matco MC-1's. Out of production, but you can still buy rebuild kits. As long as the rubber boots are in good shape, they should be serviceable. Not sure if you can hook them up in tandem to have brakes on passenger side.
For the record, I got a reply back from RotaxRick today saying he's never seen one crack this way either. He also says C Box gearboxes and parts are starting to dry up with Rotax going out of production on these and with all the Apex and RX-1 conversions going on now.
I'm watching with interest, JimChuck. I just picked up a partially completed Model IV kit. I've got to back up a little and take care of some corrosion, old fuel tanks and the like before any progress starts. It'll be interesting to see how light you'll be able to keep it.
I can try welding it, but castings are a crap shoot, especially after they're oil impregnated. First choice would be to find a used part. I could buy new from Rotax, but I'd rather light six Benjamins on fire before I hand over that much cash to the Austrians.