Quite a few people have the brace removed, replacing it with a straight metal rod and counterweight.
.30 Mauser should still be factory produced.
Not sure any do. I saw an excellent C-93 for sale a while back, still S5, despite being chambered in 7.65x25mm Borchardt (bet that's an awkward proposition to source).
Something like the Mars might qualify, seeing as they were made in various weird, proprietary calibres.