Well there are a lot of factors defining how much usable material we could get, and how hard it would be to do it.

Yeah, about 98% of the sun is hydrogen and helium, with other elements making up the remaining 2%.

The machine used to generate the magnetic field would likely be a ring rather than plate, with the goal being to bend the trajectory of any matter that passes through the ring just a little. In effect it would work a lot like a lens, that could focus matter passing through it into a cone of trajectories, with collection happening at the point of the cone, possibly a point at a much higher in orbit. (This does introduce some complications in the different orbital speeds for the ring and collector, but without getting into it, there is a solution for that, it’s not the hardest part of this idea)

And how much you can capture depends a lot on how close to the sun you can put your magnet field ring. If it’s stationed closer to the sun it shrinks the size of the sphere you’re trying to cover. So if your ring could survive at 0.2 AU from the sun (about half the distance of mercury’s orbit), a ring of the same diameter would cover 25 times more area of the sphere than if it was stationed at 1 AU.

So your 59.5 tons collected turns into 1487.5 tons, 2% of which is 29.75 tons of usable material (which I’ll be honest, is not great considering the magnitude of the construction project). It’s probably a better deal if you’re using the hydrogen towards fusion power, but it’s still not great.

The good news is that it scales well, the larger you make the ring, the better your ratio of materials gathered vs materials needed to build the ring, which makes the optimal size of a ring one that basically circles the sun. So… yeah, this is not a project in our immediate future.