Like Deadshot said, unless you get a donor action with the magnum bf, you'll use the action body. Or, you can have the bolt face machined to magnum dimensions, then have a new (external) extractor installed, and trued with the action. Start your build from there. with the cost of the machine work, you'll be a good part of the way toward the cost of a new PT&G bolt.
With the PT&G bolt, you can get everything you want, with any of their options, sized (dia.) to fit your needs, plus the benefit of a 1-piece bolt. There will still need to be some minor fitting. You can sell the bolt from the donor action to offset a portion of the cost of the new bolt.
Compare the cost of that process vs. a custom action, and see which one fits your budget best.