but am not sure how to calculate current through the zener to select the appropriate part?
Its not so easy.
It all depends on your inductive load of your Injector.
Lets assume you have a injector with ~12 Ohms and ~10mH inductivity. (may vary in your case).
the initial zener current on switching off is then the Injector current ~1.2A @ 14.4V.
So a Zener of 51V and 1.2A current would theoretically give a ~62W Zener diode.
But the zener is only loaded on switch off.
So you have to regard the switch off energy.
E = 0.5 x L x I x I = 0.5 x 10 mH * 1.2A * 1.2 A = 7.2 mJ
At 6000 RPM and 4 stroke engine this happens 50 times / s giving a average power of 0.36 W.
So you need a zener with minimum 0.5W and pulse capability of 7.2 mJ (in warm condition).
The bad news: these pulse parameters are usually not specified for a normal zener.
So you will have to use a much larger zener as calculated for average power.
with best regards
Andreas