require_once(CRM/Core/Payment/SecurePay.php) failed to open stream: No such file or directory in Civi\Payment\System::getByProcessor()
Using a library that is not customised for CiviCRM will not work. Maybe the pre-upgrade system (the 3.4.8 version) had a custom made payment processor for SecurePay. Look if you can find the file SecurePay.php in the backup of the old installation.