The registration page is one of only a few "real" physical aspx pages in mojoPortal. If you want to really customize it, and are using mojoPortal 126.96.36.199 or later, you can specify a custom page name and enable redirecting in your user.config or web.config like this (lifted straight out of web.config):
<!-- this key makes the Register link point to a custom registration page. leave blank to use the built in registration page.
<add key="CustomRegistrationPage" value="/path/to/your/customregistration.aspx"/>
Set RedirectRegistrationPageToCustomPage to true to force a redirect from the built in registration page to the custom url
<add key="CustomRegistrationPage" value=""/>
<add key="RedirectRegistrationPageToCustomPage" value="false"/>
You can also force a redirect to a post-registration page, if you want to provide a particular post-registration experience:
<!-- Generally you should leave this blank and let mojoportal handle the redirecting after a
user registers. However, if you want to force redirecting to some specific page you can
force it with this setting (e.g. <add key="PageToRedirectToAfterRegistration" value="/yourcustompage.aspx" />).
Warning! Do not use this in a multi site installation. This is a global setting and will
affect all sites. -->
<add key="PageToRedirectToAfterRegistration" value=""/>