Maybe take both off and take them to an alternator/starter shop. They might even
rebuild the ones you have for not too much
money while you are at it although having spares is a good idea. They will be able to source you low-price
replacements. Getting the starter out is a chore though. There is a lot of tubing that needs to be removed and replaced and the entire
heat exchanger. Follow the procedure and the order of parts
removal in the manual and buy all new sealing rings for every connections as they will likely not seal again when you put them back together. Have more spares for when/if you ever need to replace the starter with the spare on the
water as well. They get hard rather quickly and will not reseal reliably. A leaky seal is the number one reason for losing the starter since it will leak right into it and destroy it in short order. It's an
extremely poor design/layout.