Although now very old, both the early distributors used on the Austin 7, the DS4 and DJ4, can be maintained in usable condition. While new capacitors, caps and points for the DJ4 can be bought from several reliable suppliers, they are not available for the DS4. However, with a little time and effort, it is possible to modify other points to fit, and the capacitor is large enough to accommodate a modern replacement inside. The cap for the DS4 can be replaced by one from a Renault Dauphine, and the rotor arm from a Vauxhall Viva HB. It's also possible to convert both to electronic ignition, as a search on YouTube will show.
Unfortunately, almost all ordinary modern capacitors, imported from China and other countries, have proved to be of a disastrously low quality, and these should only be bought from expert suppliers who offer a full rebuilding service. As an alternative, a modern and highly reliable polypropylene capacitor can be used; some of these are tiny, while others too large to fit inside the distributor and must be mounted remotely.
The easiest way to connect a larger one, for example, the 598-224PPB102K or the RIFA PME 271M (available from RS), is to the terminal on the coil that goes to the distributor and the other to a convenient earth point. The unit itself can be hidden away under the dashboard, or in a more convenient position, perhaps held with industrial-grade Velcro to the bulkhead beneath or to one side of the coil.
The tiny CBB21 (many sellers on eBay have them) is just as effective and can even be fitted inside an old Lucas capacitor if the original guts are removed. It's a good idea to then encapsulate the new one in Araldite or a similar material. However, it's been pointed out that when the points open, a back EMF of several hundred volts is developed and the capacitor should be able to handle this. The 598-224PPB102K is rated at 1000 volts and the tiny CBB21 at 630 volts, the latter the minimum that should be considered. Another option of the latter type is the Japanese-made QXK2J224KTP, a unit proved over long service in many Austin Sevens used for competition and, one more, the Swifttune, another motor-sport-proven unit.
Some of the mentioned capacitors are illustrated at the bottom of the page.
For more technical and background information on this subject see these two very interesting articles: https://www.da7c.co.uk/technical_torque_articles/capacitors.htm    and https://www.nonlintec.com/sprite/cap_failure/.

The two screws that hold the condenser on are in steel with a 5BA thread and of a very rare type - they have a hex head and screwdriver slot.
You can buy ones that need a slot cutting here:
https://www.chronos.ltd.uk/product-categ...e-ba-bolts
and here: https://www.ebay.co.uk/str/maccmodelengi...&_tab=shop
amd here: https://www.ba-bolts.co.uk/steelba.html

The DJ4 as used from early 1931 until the advent of the automatic advance and retard DK4 in 1934

Both the DS4 and DJ4 where secured to the dynamo body by a special two-diameter bolt. The bolt could be locked down tight, but this did not lock the distributor, instead a spring wrapped around it upper diameter allowed the distributor the freedom to be rotated by the hand control in the middle of the steering wheel. The system allowed a little vertical place in the assembly with the drive gear on the end of the drive shaft being to move away from its central location against the dynamo spindle mounted drive gear. This this does not seem to have affected the  performance of the ignition system.

The clamp that fastens around the distributor shaft. The slotted hole is the one through which the two-diameter bolt passes. The adjust the ignition timing, the clamp is slackened allowing the distributor to be tuned before the clamp is once again locked tight

While the DS4 had push-in HT leads, the DJ4 used screw-down caps. Both the cap and the 'mushroom-headed' retaining screws were brown