Help!!

If all four cylinders at once stopped sparking that suggests the fault lies upstream of the individual cylinders, and of their HT leads, and of the contacts in the distributor cap. If you've replaced the coil and you are getting a 12V supply to it that suggests the supply to the coil is OK. That leaves the possibility that you have TWO faulty coils - not unknown, sadly! - or the fault lies in the HT lead connecting the coil to the distributor cap.

If the cap itself has been replaced we can assume any tracking or fracture of the cap has been eliminated (unless, again, you have two faulty caps) so that leaves the HT lead which you don't mention as having been replaced. Although an HT lead can break down some where along its length, that's pretty rare unless it's been nipped or bent or trapped in some way. More likely is the pressed metal internal connectors inside each end, which are slightly sprung to help contact with the coil and dizzy cap. If one of these has fractured simply with age and vibration then you will lose a lot of the 'spark' as the current tries to jump the gap across the fracture in the connector. If the gap is too large then of course you have effectively open circuit.

Alternatively there *is* some breakdown in that HT lead and the current is simply going straight to earth and bypassing the distributor cap and plugs.

Although you said "all cabling good" you don't say how you can be sure of this if you haven't replaced the HT leads. As a first measure, you could simply clean the outside of the coil-dizzy HT lead to be sure there is no film of oil or grease on it which is leaking current to earth. Also inspect it for any previously unseen cuts or splitting of the insulation, and put a meter on it to be sure you really do have a continuous electrical path from one end to the other.