If you have a movable manual camring in your housing, the one which lacks the operating plunger and cable, then you could still use it if you ensured that the ring were jammed in its fully advanced position. (That is, with the ring rotated fully anti-clockwise against its stop looking at it from the points end).
You can do this by using a bolt with 1/2" BSC (Cycle) thread down the thread for the 'tube', to bear gently against the outside of the camring or, better, against the head of an original type of plunger (shortened to suit, with or without a suitable spring to take up any slack).
Or, you could arm yourself with the missing parts that control the camring's position, with slack cable 'full advance' henceforth being the normal position if using the ATD. Think that's what I'd do in the circs.
There are advantages to having both a manual control and the ATD. The ATD is purely rpm-controlled and is fully advanced by not much over 2000 engine rpm, thus there's no ability to retard the timing under heavy load at moderate rpm, which can sometimes reduce pinging with alloy heads & high compression, or if there's a sidecar to pull along. May never be necessary, but can be a 'nice to have'.
Alternatively, you could fit the end housing with a fixed camring if you wanted, subject to the pattern of holes for the fasteners being the same. (There are variations, depends which K2F versions the various bits are off.)
But bear in mind that mixing and matching -
* can cause minor hassle relating to armature end float, requiring some shimming adjustments which can be fiddly;
* certainly comes with the need to have extracted and swapped bearing outer races to get a matched pair for the cb end, and
* may possibly increase the risk of uneven points gaps and an unequal firing interval between cylinders.
A separate option would be to obtain a fixed drive gear and use the manual advance and retard all the time. Ie abandon the ATD.
With your rebuilt motor, prudence dictates you want the mag as good as you can possibly get it. So whichever way you go it would be wise to check the timing on both cylinders when you are done, because errors of several degrees between sides are not uncommon. (1° on the mag = 2° on the crank; 5° on the mag = Don't Even Think About It on the crank!))
I don't think you can "just leave well alone", because the essential requirement is to anchor the position of the manual camring if you use what's there already.
