In case anyone else is wondering, YES you can re-purpose a 12v "Sport" digital Lap Counter. Using a Dremel, super glue and some hot glue for cable management, I managed to make it work and put together in a robust and reliable package. It's a little fiddly and has tight tolerances but it all fits together ok and is doable in 3-4 hrs (I'm a total newbie to Scalextric). I expected problems but planning it out carefully and testing as I went meant it worked first time. I used the used unit as a guide for layout of internals, chopped out the track mouldings for switch/mounts and pivot points for the microswitch/arm, plus a screw hole from the underside cover plate, super-glueing them in place underneath. I cut away the plastic gulley (the car guide goes into) to fit the switch arm. Cable management is possible with superglue. It works reliably, is sensitive so as to never miss a lap and does not impeded the cars at all. Sorry but I've no more detailed images and probably won't be able to answer your questions, I just wanted to give something back and show its possible.
