Somewhere on the old relays the rating should be stamped/printed.
I dunno about yours, 'cos I've just got a little 40hp, but when my relays started playing up, I removed one and took it to my local friendly auto parts dept. The guy in the stores took one look at it, and immediately produced a pair off the shelf. These are obviously auto parts, not marine ones, so when I Installed 'em I simply covered 'em in silicon to waterproof 'em.
Two seasons later, they are still going strong, and if and when they need replaced, the silicon can be peeled off!
Hope this helps,
"Remember the Plug!"