The former tool to create public key for activation is missing.
This tool/page should be ported asap.

I tried to configure a app in tools by myself, but its not possible as the class and html page is not a part of the admin interface module anymore.

Possible work around:
Using a M4.5 instance to create the private & public keys, and add them manually to the right places:

• Public & Private key into file on author:
  /webapp/WEB-INF/config/default/magnolia-activation-keypair.properties
• Public key into config workspace on Author&Public instance:
  /config//server/activation/publicKey