You actually have two problems there, and both are a nightmare.
One is getting a feed from a speedo sensor which would either mount and feed from the gearbox sprocket which you would need to cobble together probably from another model, or maybe use a magnet and sensor type arrangement on a wheel, but they look a mess and are inaccurate too.
Secondly you will need a converter to go from electrical to mechanical ...they are available in the car world and run around the 300 quid mark, but they all tend to use car style speedo cables, so again you have a big problem to overcome as it wont fit your speedo.
Examples here
https://www.speedhut.com/Speedbox.html in the spares section, but there are plenty of similar if you Google them
If you dont/cant go that route, you are left with either finding a wheel with a speedo drive that fits whatever forks you have fitted, or machining a speedo drive into a wheel hub that you already have
The cheapest and easiest way is to just use a GPS speedo. That keeps you legal at least. After that, I would look for a wheel with a speedo drive. Depending on what size wheel you want, there are plenty of donors that will do that.
Me - I would (did) find a wheel with a speedo drive on it. It makes life easier.
What size front wheel are you wanting ?