I'll keep an eye out for a mounting plate for you. In the meantime, below is a photo of my old 149 with the tiller drive. You should be able to get close enough on the dimensions from this photo. You already have the most important part, the width of the mounting arms on the tiller.
If I were making one, I'd make the tongue for the hitch integral to that plate instead of bolting the factory hitch back on. If I remember correctly, that is the one piece that you have to take on and off to put the tiller on and off. The hitch just stuck out about an inch too far, and would hit the tiller, but a small tab sticking out an inch, just enough to get a pin through, would have worked and would not have been in the way.