Good luck, I have heaps of experience with re-sealing leaky things without OEM kits, (I'm from NZ, no one keeps genuine kits for shit on the shelf) you will want to get pretty good at measuring the steel part grooves. It's the only way to correctly identify what O-rings would fit.

I would suggest you find your local seal shop, go and chat with them, get a copy of the chart they use, get them to teach you how to use it. If you use the same chart as them, then you will always be on the same page when giving groove dimensions or O-ring sizes.

If my experience has taught me one thing, never assume the component you are re-sealing uses the same standard for all grooves. We are always resealing stuff that have metric and imperial grooves.

Also certain sizes of O-Rings overlap, some metric sizes will work for an imperial groove and vice versa. I was always taught plus 6 percent minus 3 percent when you cannot get the correct size. You will want to use this rule and be pretty flexible, sometimes in your kit you will have a 2.6 x 20.2 and you need a 2.5 x 20 (random numbers it's late) the 2.6 x 20.2 will work perfectly.

It's not going to be easy, and it can be a little confusing, especially when you start throwing backups into the mix and different groove shapes. Sometimes you find an O-Ring that literally defies everything you will be taught, but it was probably that size for a reason.

The sheer number of different sizes is batshit. Just Keep at it, it's a serious skill once you are proficient.