Change Payment Gateway Script to work with modified OSCommerce

We run a very modified version Oscommerce and changed our payment accquirer. Of course this one uses it's own script to communicate the payment to the server of concardis (the provider of the payengine - also known as ogone). This script does not run in our environment, i think the store has been modified so much that it can't work. The problem: While the checkout process it should open up a popup to fill in creditcard / bankaccount data - process the payment over communication with the server and then send the feedback if the payment has been successfull, but the popup does not open and the order runs through as successfull. Your task would be: 1. Check why the script does not work on our test environment. 2. Modify the paymentscript and/or the shopsscript to get this payment script to work. 3. All modifications have to be documented. 3. Test it in the testmode. Thank you for your biddings.