The V belt pulley is supplied by the mower manufacturer, not Briggs, so there is not a standard way to attach it. There will be a key between the pulley and the crankshaft, and a grub screw or something else to locate the pulley vertically. (It could be a screw axially in the end of the crankshaft.) Remove whatever it is, apply some Penetrene to the shaft above the pulley so it soaks into the joint, wait a couple of hours, and apply a gear puller. If it is a large pulley and therefore not hugely robust, and if it is really tight, supply pictures and we'll discuss solutions. Large pulleys break if you try to pull them with a gear puller.

Here is the instruction from Briggs for installing that seal:
http://recoveryvehicles.tpub.com/TM-5-4240-501-14P/css/TM-5-4240-501-14P_259.htm

Some mechanics remove the seal by using a hook behind the front part of the seal outer (not the sealing lip: never try to get behind that). The new seal can then be tapped into place carefully. However I think most dealer mechanics would prefer to remove the crankcase cover and do the job on the bench: outcomes are more assured that way.