PowerApps Portals licensing details are available here, including a comparison with Dynamics 365 Portals.
Starting soon, if not already now, the Dynamics 365 Marketing setup wizard will be able to set up and integrate with a free demo PowerApps Portal when you install Marketing. The demo ends in 30 days, after which the customer can sign up for a standard, consumption-based license. The Dynamics 365 Portal product is to be discontinued, but customers that already have one can continue to use it under the old licensing terms; and that includes setting up a new Dynamics 365 Marketing instance to use an existing Dynamics 365 Portals entitlement (provided it isn't already in use by another app or instance).
As far as I know, the events website will continue to be delivered as an Angular project, which can be installed on a PowerApps Portal, or an external site, and customized exactly as before. I don't think you'll be able to use PowerApps design studio to customize the Angular-based events website, but I am not completely sure because I have not worked with the design studio product.