you can get by with an ignition condenser of a similar size..
With the average ignition condenser, physical size is closely related to the condenser's capacity. Substitutions could be made on size alone and are probably gonna work fine.
Particular capacity doesnt matter a lot but it does matter if the points spark a lot and are wearing quickly..
When points "burn", some material from one contact is being transfered to the other contact. If you look at the points contacts under a microscope, you could see in which direction the material is going. One point will lose material and be pitted with holes.. the other will have tiny mounds growing on it.
If material is being transfered from the negative to the positive point contact, increasing the size (capacitance) of the condenser would be called for... and vise versa.