This probably isnt too difficult once you find what you need. I assume the speedo cable screws into the t-case, dana 20? With some google searching you can probably find the answers you need and I doubt you are the first one to convert mechanical to electronic speedo. There were a lot of speed sensor apps that simply mounted the speed sensor to the same type housing that cable speedos used. I bet you can find something like this that will fit on your application. My 95 ZJ uses this type speed sensor and many jeeps did too through the 90's, GM used this setup too through the 90's.
I dont know if that makes any sense to you or not. Basically if you can find a few housings to pull off of donor vehicles, you might find one that you can put your gear on and use the electronic speed sensor to run your speedo.