Is it leaking because it's not been run for some time and the sump is full of oil?
I have only replaced this when I was doing the cam change and had the carrier plate off, it was easy to knock out from behind. I would think that it could be replaced without removing the plate as long as you can get the old seal out, there are a number of seal pullers on eBay and I have done some similar seals by drilling the face and using a self tapper screw each side to pull on. There are usually numbers embossed into the seal front face to show the size, or measure as Rog and Phil have said. Or, looking in the parts book it says, "Seal, pinion, dual lip, 22mmx35mmx7mm" p/n 106-0733, even easier
