Steering stem can be pressed out of the SP1/SP2 lower clamp.
Take it to a machine shop with a press.
Note: Check your offset. I was going to do a swap for my NC30 but found out my stock offset is 35mm from centerline of steering head to centerline of forks. SP1/929/954 offset is 30mm. 5mm difference in offset is going to upset the handling of your bike.
NC30 has some parts in common to the RC30. Parts Classification Number, or PCN is the same:
91016-MR7-003 Steering head bearings, upper and lower
53214-MR8-003 Upper steering head dust seal
Lower dust seal is different.
90304-MR7-000 NUT, STEERING STEM
SP2 share the same parts. Check ronayers.com for the fiche. Using a SP2 upper and lower clamp with RC30 steering stem might be your best bet if you want to go the economic route. Although, you would have to figure out how to mount the SP2 lower clamp to the RC30 stem. I haven't checked the Durbahn site so I don't know.
Perhaps he had to use the RC30 stem because the SP2 stem was too long to fit in the RC30 frame. Which is the case with the NC30. I think the reason why he used the RC30 stem is so he didn't have to modify the SP2 stem. The RC30 stem should be the same diameter as the SP2 stem. So you can mount the SP2 top clamp/bridge to it.
My NC30 lower clamp and stem was welded together. Also, the steering stem was about 50mm shorter than the 954 stem. I had to either shorten and rethread the stem or use a big spacer. What did I do? I went with adjustable Attack triple clamps with correct 35mm offset.
Check out Robs website. It'll give you some useful information.