I happen to have exactly what you're looking for, almost. It's a 3x4 speed stripped down to box and shutter to use on my Eastman 2D only I never got around to it. I came across it the other day in a box of parts. It works fairly well but gets slow at the low tension setting and getting down to the T end of the curtain. PM if you're interested.
I have three 3x4 SG for sale but none are junkers. Whatever you buy I strongly suspect the shutter assembly will need servicing (usually just a clean and lube). It's not a difficult task... just needs to be done.