Redirecting error 404 can be solved with a little javascript code adjustment in page 404.php.
Some browsers are not supporting javascript or user disabled it. When that happens a link will be displayed to your search page instead...
Copy and insert before line <div class="error404">
Replace yourdomain with yours:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" dir="ltr" lang="<?php echo str_replace('_', '-', osc_current_user_locale()); ?>">
<head>
<?php osc_current_web_theme_path('head.php') ; ?>
<meta name="robots" content="noindex, nofollow" />
<meta name="googlebot" content="noindex, nofollow" />
<meta http-equiv="refresh" content="0; url=https://yourdomain/search">
<script type="text/javascript">
window.location.href = "https://yourdomain/search"
</script>
</head>
<body id="p404">
<?php osc_current_web_theme_path('header.php') ; ?>
<h1>If you are not redirected automatically, follow this <a href='https://yourdomain/search'>link to visit the website.</a></h1>
<div class="container">