I don't know if the drive units are interchangeable but they both have a shaft, maybe you can graft those together?
Pull both the mechanical speedo drive and the electronic drive out and compare them, sometimes the shafts only need the actual nylon gear that drives the shaft to be changed. I have had to swap these when changing for different g/boxes that both have mech drive, but haven't really tried electto mech.
You pull out the cone shaped section out of the mechanical one and can then diconnect the spring clip that holds the shaft in. These shafts are available at toyota seperately, so maybe you can, with some fiddling, swap the parts?
It's always the bloody small things that suck in a conversion!!!
How'd you go apart from this. By the way, I'm about to/during/half completed an intercooler change if you want pics of an actual Trust/Greddy type unit to suit as a bolt in, maybe you could get a core made from the specs etc?