We have repaired speedos for years.
We periodically get junk mail describing various speedo related functions. Recently we got a flier for a device that converts electrical speedo signals to drive a small motor that spins a short speedo cable at the proper speed.
My brother does the speedos and I gave him the flier. When he gets here I'll see if he saved it. Otherwise you might try a speed equiptment vender for such a product.
Bosch Master, ASE Master, L1
33 years MB technician