Couple of things: If you end up having to use industrial screws, you're fine as long as you use high strength fasteners. Bolts, for example, come in Grade 8 for strength. Screws are also available in high-strength options, more than adequate for this. You didn't post a picture, and I'm no authority on Mausers, but the receiver/trigger guard/barrel bolts I've seen have
oval head countersunk heads. I seem to recall a discussion on another forum about Paul Mauser using
British thread forms on barrels (don't quote me on this), so don't assume they're either unified or metric threads. Another thought: Outfits like Numrich sell hardware for other models of Mauser. That might be a starting point for you. They show Mauser bolts, long, fully-threaded, possibly the right thread and head form for your rifle, that you could modify. Just some ideas to ponder. Sending you a PM.