The two currently available and simple ones are made by PowerSpark and Accuspark, about £40 each. Both look and perform the same AFIK but the D300 distributor in the Slant 4 needs to be fully dismantled to fit them. I believe that Luminition kits are still available but very much more expensive than the above ones.
If you can find one, the full Lucas electronic distributor from a late CF van is the preferred route for competition engines. These usually command anything up to £200 for a box of bits to assemble and design the wiring yourself....not for the faint hearted!
Don't forget that the HC range has a resistance wire in the ignition circuit which doesn't like eletronic ignition systems and needs to be bypassed for safety and reliability reasons. You'll also need to fit a non-ballast type coil to complete the package.
See my thread below.
viewtopic.php?f=12&t=22328