Geoff, At the moment the choice is somewhat limited, and it has been that way for quite some time, there have been a few attempts designing an installing an electric starter to a Twin Vincent, but the only one that has had any real success and longevity is the Francois Grosset starter, lots of info on this forum if you use the search icon. I have used one for several years and have been very pleased with it.
There is talk of Herve and Paul Hamon (of Alton fame) designing and manufacturing a starter system for the twin, but I do not know anything about the progress of that venture.