Nic, with a 21.1 steerer tube, you're much better off just getting a new used fork with a more standard inside diameter. The bike will be using the "normal" 1" headtube so as long as you find another threaded fork that has the right steerer tube length you should be good to go. If you REALLY wanted to "do it right" you could find out if your headset is ISO or JIS and get a fork/headset that would fit appropriately, but the difference IIRC is only 27.0 vs. 26.8 and either really works just fine (I have one of each and have them running opposite for winter).
If someone tells you to ream your headtube out to solve this problem, don't listen.
Also, I fathom you're probably trying to build on a budget and getting a new (used) fork probably is an expense you hadn't anticipated, but realistically you're going to spend more money trying to source fringe parts for a crappy frame. Your best bet is of course to find a more suitable frameset (perhaps easier said than done). Either way, it's worth considering that even if you found parts, they'll most likely be pretty low end.