Not sure of an exact size but you will easily be able to get another from SKF or one of the bearing/seal manufacturers or even a local garage.
Something close would be fine as it is only a sealing washer that puts even pressure on the rubberised "O" ring instead of letting the end of the spring damage the seal.
You could if really really desperate (and broken down in the middle of the desert)
go without it but how much exactly do you like removing your carbs really? and you could expect the seal to be wrecked pretty quickly by the unprotected end of the spring.