Congrats on the buy, if you are a do-it-your-selfer, then the car will work out good for you.
Our Capri shares many parts with the 1989 Mazda 323, and lots of folks sell Mazda distributor o-rings, should work OK.
You will find many parts for the 1989 Mazda 323 that will work on the Capri (like the gas tank), but many folks (like Autozone) don't cross reference it into the Capri.
Are you sure it's the o-ring? Valve cover gasket would leak more than an o-ring, and its right there at the distributor...Chris