most modern ignition systems come with a spare lead just for that,
RCXCEL IGNITIONS http://www.petenplanes.com.au/engine.../prod_811.html
3W ENGINES http://www.aircraftinternational.com...4/Default.aspx
the best tachs are
http://www.fromeco.org/products/05frctnc/ they work in any light basically
i use this one and its not quite as accurate on cloudy days or when the sun is going down
http://www.hobbico.com/fieldequip/hcap0401.html