Is it one of the rebadged Yamaha motors? If so, I'd think the O-rings would be metric along with the hardware.
Shouldn't be too hard to find suitable replacements though: Put a caliper on the groove to get the ID, and then on the O-ring itself (or what's left of it) to get the thickness of the material. From there, you should be able to match up with O-ring specs online. You'll have to do a bit of guesswork on the missing one, but you're just isolating reservoirs of hydraulic fluid, so it's not all that critical.