OK, i dont know how the EK Si cluster is setup, but with my EF Si cluster, you can remove 4 little screws in the back, and two tiny screws by the Speedometer needle. If you pop the housing open and inscrew this, you can take out just the speedometer and swap it with your stock one, without having to swap the whole gauge.
Hope it helps you out.
If it doesnt try to find a shady used car dealership and ask if they could do it for you.
All you have to do is take the odometer from your original cluster and put it into the si cluster. i have done it to mine and my friends.
first take off the needle
then take out the two screws on each side of the needle
next take off four silver screws from back of cluster.
the odometer will come right out
then swap them out and put them both back together in reverse order