To clarify MVC does not use controls ServerControls or UserControls, so you cannot re-use much of anything from WebForms in MVC. That is what is incompatible about it. WebForms builds a tree of controls even Page is a control in that regard and each control in the tree participates in an event lifecycle ie OnInit, PageLoad, PreRender, Render and these events are fired recursively through the tree down through child controls.
MVC has none of this so you cannot build a component in MVC and plug it into WebForms nor can you build something in WebForms and plug it into MVC.
You can have WebForms pages and MVC pages side by side in the same application but they cannot integrate together very well. We already have a plugin model for routing and one could plugin MVC pages and configure the routing for them but they will not use the same skinning model and therefore will not be visually integrated. In fact the term pages is misleading because there are not pages in MVC in the same sense that there are pages in WebForms that inherit from System.Web.UI.Page. MVC has routes and handlers and such but not pages in the same sense as we use the word in WebForms.
One could build an MVC app and run it in their mojoPortal installation but it will not be visually integrated and it will not be able to re-use any controls.
WebForms is a much easier model for developers because it does a lot for you so you don't have to already be an expert in http, html, javascript, and it simulates a stateful environment with events whereas in reality the web is not a stateful thing it is just a request/response kind of thing where each request is like a brand new request. By things like viewstate and the postback model WebForms gives us a stateful event driven paradigm that is easier to understand.
It amazes me how many people say they want MVC and don't really even understand what it is or how it differs from WebForms. MVC (Model View Controller) is just a design pattern. There are some good things about MVC, it promotes separation of concerns and makes a more testable architecture for those who do test driven development but those things can also be achieved in WebForms. If test driven development is important to you it can be done the the WebForms MVP (Model View Presenter) framework while still keeping the advantages of WebForms. WebForms is more approachable for new developers and more geared towards rapid development. End users don't care whether an app is built on MVC or WebForms they care about the features and the value delivered by the application. There is no reason to rewrite a well implemented WebForms app in MVC especially a large one that has been evolving for 7 years and already delivers a lot of features and value.