So like it says in the topic, I have an Arsenal SGL-21 and I'd like to change out the muzzle brake for a slant brake. I understand I need to remove the FSB and replace it. Can I replace it with one that has the correct threading for a slant break? Is a press required for this project? I'm also thinking about having the barrel chopped or going 14.5 or whatever the AK equivalent is of that with a pinned muzzle device. That is obviously something I'll need to send it off to get done. Any recommendations?

Slant brakes thread directly to the barrel so you'll need an AKM front sight block and to have the barrel threaded to 14x1LH.

As was stated,  you needed the akm fsb and have Barrel threaded. I wouldn't bother of it were mine though.  Slant brakes do nothing in semi. If the blast from the current brake bothers you, replace it with a flash suppressor. That's just me though...

Not sure bbl diameter is big enough for 14mm threads under the fsb. 1/2" yes. If your sending it out to a smith for a chop, he can make it happen.