Its a tight fit, the pulley may have enough room to slide off the crank without removing the front end. You may have to loosen the front end to give yourself a little more distance between the pully and the front end axle pivot assembly. I took mine completly apart the rebuild it, so I cant say for sure if the pulley will come off without more disassembly. Once you get the pulley off, inspect the sealing surface for grooves.
I did learn one thing about putting the new seal in. You have to make sure you press it in far enough or it will bind on the pulley when you tighten it down. Heres a few pics if they help. One is the timing cover already off of the Perkins gas, you can see the the front seal. Another is with the motor reassembled with the pulley on. Another is with the front end already off, before I rebuilt it.