capacitance is determined by how many square inches of 'plates' are in the capacitor.. And, ignition-capacitors have something like a 20% tolerance.. and caps are virtually identical on the inside: two plates, actually thin strips of metal, separated by a dielectric.
so, if you pick some ignition-type capacitor close to the same size, it will be OK. Just be sure it fits in the space provided for it in the magneto (if thats where it goes).
As far as breakdown voltage, use at least (?) a 6volt (i don't think they make one for ignitions thats rated lower).. a 12 volt is ok.
personally, i wouldn't hesitate to snatch one at random off an old car in a junk yard.