The capacitor in a Lucas magnet is somewhere deep in his guts. That location is not optimal in terms of temperature and such. This results in starting problems.
Brightsparkmagnetos makes everything for Lucas stuff. The the Easy Cap Condenser, is a ceramic capacitor that is easy to assemble and that greatly promotes starting behavior.
The thing costs around 25 pounds from the factory, then the shipping is added
www.brightsparkmagnetos.com