How far do you go with this is the main question.
Chain & Cable assemblies - at least one of each (and bear in mind that they are probably different for the different versions of drive so you may end up needing to take 2 assemblies in total - I think they are the same for front & rear - if not you will need 4)
Idler cables (as above - you probably need 2)
Pedal shaft/pedal shaft assemblies (if you take just the shaft all you get is the pedal on the shaft - to actually install that on a drive you need to transfer the leg-length adjustment mechanism to the new shaft for which you need a very small allen key (to remove the retaining pin) so add that to your toolbox. Or you could make up full pedal shaft assemblies before you go... but you can bet that you will still need that allen key even if you do... e.g. to transfer an adjuster to a different pedal shaft). In extremis the pedal assemblies can be used on either side but on the 'wrong' side your foot will be pressing on the back of the pedal which is not terribly comfortable but better than paddling.
Fin-masts or mast sprocket assemblies. In my experience you are unlikely to break a fin mast if you are using the shorter fins (either sort) but if you are using the ST Turbo fins a fin-mast breakage or 2 (across your 4 boats) should be anticipated in your planning.
Spare Fins (one at least - just in case).
Rudder pins - usually these are clipped into the twistlock hatch covers: best make sure they are present.)
Tools to enable you to service the drives.
Knowledge to enable you to troubleshoot, service and adjust the drives.
Some strong line, epoxy glue, swiss-army penknife and some No. 9 fencing wire.
Stow this stuff towards the end, after you have stowed everything else apart from your baler, radio and
epirb (then it will be closer to hand should you need it). And don't forget that Murphy's law will apply i.e. that if you don't take it you will end up needing it and if you do take it you will end up not needing it
Edit: okay found out what it is.