I'm sure I've got one I can spare in the shed. The actual value probably isn't critical. An ignition condenser (capacitor)off the dizzy would likely do the job to.

If I remember right it's for noise suppression and probably the bigger the better (with diminishing returns). I think the brushes on the rotor cause the electrical noise. One of the elec engs I work with would probably know. (Beside the point I guess.)