There is a more 'expensive' kit to fit directly to a 126 Bis, from
http://www.distributordoctor.com/I emailed the owner {Martin Jay] recently, and he has a kit for a Hall effect sensor, rotor, etc and fitting kit, for £131.82 + vat.
I suspect it will be made up by him to suit.
I have used one of his kits on a Ford sidevalve engine of mine...and it is excellent.
The kit comes with hall sensor, ring, and mounting bracketry, probably meaning, a new base plate...all ready to fit.
{Certainly that's what I got for my Ford engine]....although the Ford item didn't cost as much as the above 126 ....probably a case of, numbers?
Good service, IMHO....and the items are top quality. [including some heat resistant grease for insulation...important on the Ford]
This firm also has produced [ in the uk] their own rotor arms for many applications, given the rubbish that has been coming out of China these past decades, for older vehicles]...with their high carbon cheap plastics. [leading to mysterious arcing & misfires]
If going the route of a crank sensor ..then it is possible to fit a completely mappable module, thereby tailoring the advance curve to entirely suit one's engine, with al its mods [Megajolt, for example??}
But that involves laptops, something I'm not entirely comfortable with.