Remove the powerhead and change it.
Yes......
As Haffiman indicated, you may find a couple of powerhead base gaskets for that application. I've seen regular paper gaskets which won't last long before they fail. The more desirable gasket is a thicker one, with an aluminum core sandwiched between two paper gaskets. That one is robust and will provide reliable sealing for a much longer time.
Should be able to get it at any Evinrude dlr.