As above, it's not as simple as some assume. For Mag21s it genuinely is better to get a replacement crown and steerer. These can occasionally be picked up for very little. Also, your threadless is quite attractive to people as the majority of Mag21s were threaded. Although this may slow down your build, I would go on here and ask about swaps.
1 as FC rightly says, check that the quill stem will actually fit in the steerer.
2 you have to check that the wall diameter of the tubing is adequate, otherwise the steerer will be compromised. This will need a bit of digging, since RS varied spec quite a bit as they went for lighter, stronger etc. Vernier to measure the wall thickness then some research on web needed.
3 steerers are TOUGH, some are VERY TOUGH. I had a problem of extending a thread on a Reynolds steerer and a highly competent tech friend said 'forget it...that will wreck any cutter which I have...'. And that was just extending a thread a bit, not cutting a completely new one. It can be done, but you need the right tools and recognise that, factory fabrication, they are cut on lathes with mega-tough tungsten-headed cutting tools. Hand tools are definitely not the same. Mild steel yes. Tempered high tensile, no.
I would def go down the enchange route, and two people will get want they want. I hope it works out, I have been there many times trying to sort things like this out and you need a Plan A, Plan B and Plan Z as the constraints become clear.
