There was not a crank sensor on the 50, at least not on one of this age.
I am going to check resistance on all coils, pulsar, charge and ignition before I fit a spare ecu. Trouble is, how do you know if a spare ecu is ok, usually parts that fail do so with a common problem.
On the USA message boards, these CDI packs are occasionally known to give out, there is quite a big aftermarket scene for these. They wouldn't make them, if none ever failed.
I have tried everything so far and spoken with others who know their stuff. Everyone's stumped. I think it is ignition and will report back if I find it.