If you cannot get a replacement then it is quite simple to convert the K11 to run an external coil driver.
This image:
http://www.fusion-motorsport.co.uk/images/k11dist/IMG_0965-1.jpg
Shows the inside of the pickup in the distributor.
Blue area is the optical pickup. Red area is the ignition coil driver. The pink area within is the actual ignition driver transistor.
Anyway all you need to do is extend the spade terminal coil wire from the coil (the spade terminal is visible on the right). This needs to be taken outside the distributor. You also need a good earth from within the distributor. This you can take from one of the screw terminals that holds the ignition module in the distributor. These are then both taken outside. So you have the coil wire and an earth. You can see two pins going to the ignition side of the module shown in the image. With the cars ignition on, one of these will be showing 12v, the other will be 0v. The 0v one is the wire that goes to your ECU from the ignition output, this needs to be cut.
So what your left with now is a ground, the ignition trigger and the coil wire. Then its a case of getting an ignition driver such as one used on any Nissan with external coil, and wiring it in to these 3 wires. I'm not going to give it all away you can work out the rest. But this is to show that you don't need to keep to the driver that is built within the distributor.
Ed