My head leaks
Two things to consider in favor of buying the kit.
First, in order to buy only the parts that are “worn out”, you have to tear the head down, and determine what needs replacing. Then in the middle of the job, you have to make a trip to your marine store to buy the individual parts. Assuming the have all the individual parts (good luck on that) you then have to go back and finish the job. I prefer to get in get out without at trip in the middle of the project. This can make a 2-hour job a 2-day job.
Second, if some parts are worn out, how many more are about to go? You could be tearing into your head in another 6 months. Not a job to be relished.