I recommend you never modify the mojoportal code directly. You can create a custom login page in a separate project and use a post build event to copy it up to the main web project overwriting the existing one. Do not modify the Web/Controls/SiteLogin.cs. As soon as you start modifying the mojoportal code you are making a fork and that makes it impossible to upgrade to new versions of mojoPortal without losing your changes. There will be bugs fixed, there will be new features you want, do not think its unimportant to be able to upgrade, it will come back to bite you.
As far as how to programatically login a user, do some searching and reading on asp.net forms authentication, there are 2 ways to set the authentication cookie FormsAuthentication.SetAuthCookie and FormsAuthentication.RedirectFromLogin
Hope it helps,