Hiya,
the original Reliant one runs off the gearbox & is calibrated to read correctly with the original sized rear wheels & tyres. Presumably you're now running larger wheels? Certain makes of speedo can be dismantled & recalibrated, (by changing the size of the drive gears inside it), to read properly again. I had the gauge in my truck done about 6 months ago by a company called Speedy Cables in Wales. They specialise in Smiths speedos, which were used in lots of older British cars.
The simplest way would be to use an aftermarket motorcycle speedo though, running off the front wheel. I'm assuming you've got a standard bike front end of some description? The original bike speedo, or a small chrome one from any of the custom bike parts suppliers, (like Custom Fasteners
http://www.custom-fasteners.co.uk/ ),
will do the job, connected to your original bike speedo drive in the hub of the front wheel via a cable.
Or you could use a digital bicycle speedo, which clips to your handlebars & uses a magnet stuck to the front wheel rim with an electronic sensor on the fork leg to count the number of revolutions of the wheel. Simple to fit & they even make cable less versions, although motorcycle engines can sometimes interfere with the signal.
You could use a GPS tracker speedo app' on your phone with the phone held in a clip on the trike, but I'm not sure if that complies with MOT requirements.
