Start with a file, if you have a small wetstone would be nice.
The file will get the edge off and you the assess it at that point.

From the looks of it once you have filed it the best you can so it will not cut the new seal, install the seal shallow so the lip will run out on the undamaged area.

Cheers, Charlie
If It ain't broke, Modify it!
87 Montero turbo Converted back in Spring1989
95 Montero SR 3.8 DOHC Only one?
93 Pajero 3 door 6G75 Mivec with paddle shifted 5 speed
Then a Gen2 SR with full coil independent suspension.