Hi Jonny,
Not meaning to sound flippant but if you ask this mob then suspect everyone who has had an issue with one bit if kit will suggest you carry at least one of them as a spare.
We burnt out two starter motors a few years back. Have carried a spare ever since.
Lost some tiny delrin balls once from one reticulating mastrack car - and carried a pack if new ones ever since.
Sods law will say once you've got spare you'll not need to use them, so IMHO you choose what you can afford to carry and hope this law works for you also.
Good luck
JOHN