Petronix if your existing distributor is in good condition (no shaft wobble). Also if you want to retain the look of the "stock" distributor..
A stock ZX 1, I'd only bother with a rebuilt unit (can get them on eBay US). Since many are stuffed by now as photo above shows.
123ignition would probably be ideal there was also someone offering similar for Datsun's in Australia recently.
Refer to here.
Here is the company that makes those CAS based distributors.
It could be your points if they are out of adjustment or are worn particularly if your feeding full 12v through them. I get rid of points as fast as I can. I use Pertronix modules and have found them very stable and ‘set and forgot’ in comparison to points.
I normally check the following when I am diagnosing ignition problems:
initial timing and behaviour of timing advance
spark plug condition, gap and heat range
Your cam may be limiting performance also. I run a hotter-than-mild cam, Pertronix, MSD 6AL and plenty of fuel delivery verified by my WBO2 and experience the ‘hard wall’ just over 6500rpm....but that is intentional by design.