Custom osCommerce-PayPal Integration (IPN)

We have a customized osCommerce system (originally based off of CRE Loaded 6.15). osCommerce continues to fail its integration with PayPal. Though it sends the correct sales information and pricing, PayPal does not send confirmation of the successful payment back to osCommerce, nor does any IPN information get stored in osCommerce. This payment confirmation must be received, and it should change the status of the order, which allows the order to be downloaded into the order processing batch. NOTES: the basket system has been completely replaced, and the purchasing system is now SKU - not product - based. As we are heading into the holiday season, we require a quick turn-around time for this project.