It is an easy fix but a long one.
I cheat by standing the mower vertical.
Remove the pulleys, polish the shaft shinny drive a couple of self tappers into the old seal and pull it out.
Measure the depth of the old seal before you pull it.
Grease the seal with vasolene wrap the shaft with cling wrap to protect the seal from the sharp edge of the keyway.
Then tap the new seal home.
I do a lot so have drifts but it is easy to do with a hammer & punch.
The long way is to pull the engine and work on it upside down.
The only real difference is upside down you can pull the seal with a seal puller or screwdriver.