Personally, I normally prise oil seals out with a large flat bladed screwdriver ( Gently !! )
It'll FUBAR the seal but should not be a problem as a new seal will come with the headgasket set, in any event, should be replaced as a matter of course
If you don't feel that confident !!, gently tap the camshaft forward a little & the oil seal should come out with it .
The CVH cam can be a little bit of a devil to get out once the cam is clear of the bearing journals.
Just be patient, keep the cam level & square to ensure it will pass through the next set of journals without snagging, ( Probaly need to rotate the cam a little too ) .
One last thing, never tap or hit the cam with anything solid, use a copper or plastic mallett.
If you don't have one, a small piece of hardwood used as a drift will do no damage
