Unfortunately, Opera doesn't allow you to change the new tab/start page. The best workaround is to Pin Momentum on Opera's toolbar, so it's always just a click away.
To add this shortcut, click the Box icon near the top right of the Opera toolbar, find Momentum on the list and then click the Pin icon next to it. This will add the Momentum logo to your toolbar, clicking the logo will open Momentum in a new tab.
