Hi Tony,
What you need to do is wrap your custom stuff in an <asp:Panel and set the DefaultButton property on your panel to the id of your "Go" button.
Then if the cursor is anywhere within your panel the enter key will result in your button being clicked instead of the mojoportal search button.
Alternatively if you can set a custom layout.master for your custom page that doesn't include the SearchInput control. Or set the linkonly property on the SearchInput to true and it will only display a link to the search page.
Hope it helps,
Joe