If you bugger the cam shaft seal on the sharp shoulder of the cam during installation, here's how to replace it.

A close up view of the back side of the seal sitting in it's recess.

Get a large shank flat blade screw driver...

and remove the 4 screws holding the ignition module base plate to the distributor cover.

Use the screw driver to pop the old seal out.

You can see the damage to the inner lip of the seal in this picture. The seal is a "fractional" metric seal and hence not available aftermarket. Dimensions are 15 X 30.1 X 7, available from URAL dealers only.

Place the new seal square on top of its recess and seat it using the 22 mm socket and light taps with a hammer.