A cheap tap and die set is probably less than you'll end up spending on a comparably nice stem. Those Nitto's are really nice.
I just did this to a stem and it wasn't that hard. Fortunately my dad is one of those people with a tap and die set and a grinder. You can cut the stem with a hacksaw or a Dremmel tool (I think that's what we used). The Dremmel makes it fairly easy to mimic the angle of the cut (hack saw tends to slip around). You can just use a file to clean up the cut, though a grinder is nice.