Kick-start shaft

Later boxes had a  different spring , shaft and  inner cover to early boxes .to make  the KS return action stronger,    it seems like you have a mixture of parts .    (edited )

Drilling a new hole could be a problem as the shaft will be case hardened, you risk going down a rabbit hole with more parts but try using this page from Andover Norton to identify the right spring and related parts (it shows 2 springs related to hook position).

https://andover-norton.co.uk/en/shop-drawing/885/amc-gearbox

Parts 72 and 72A are the 2 springs

I have no knowledge of these earlier parts, just good at delving into detail.  

I doubt it possible to mix and match as the spring will either break or bind on the shaft.,The inner cover type that you have should be your starting point . Early ones have a hole for the spring to fit into. Later ones have a stud  that  the spring hooks onto. The shaft and spring then have to match the cover. I think (not sure) that the shaft hole position is diferent on each type.  The later type (hook on) is  a better job and gives a more positive return of the Kickstart, If the bush is worn the early type tends to make  the kS to flick back as you accelerate.

Interesting content, thanks Robert. Would this be a/the difference between N and NA prefix boxes? Or is there more to it?

I have a pile of bits, and both shells. 

Back in 1965, when I was refurbishing my 1959 99, the kickstart spring had broken so I bought a new one from Lawton & Wilson (My local supplier in Southampton)  The new one was very similar but didn't engage with the kickstart shaft drilled hole.  My Dad ground off the outer case-hardening and drilled a new hole to suit the spring.  Both springs were essentially the same shape - not like the two photos where one has a large hook.