line 316 and 317 passes the product name:
$PP_DirectPayment_itemized[0][$nextIndex] = "Name" . strval($cartIndex+1) . "";
$PP_DirectPayment_itemized[1][$nextIndex] = "".$siteCart->DisplayInfo("Name") ."";
you could concatenate the SKU:
$PP_DirectPayment_itemized[0][$nextIndex] = "Name" . strval($cartIndex+1) . "";
$PP_DirectPayment_itemized[1][$nextIndex] = "".$siteCart->DisplayInfo("Name") ." SKU: ".$siteCart->DisplayInfo("SKU")."";
NOTE: this using paypal direct payments. you also have paypal express checkout. there will be a similar line on the checkout and pp_confirm pages that passes the product name, you can use the same technique to concatenate the SKU on the checkout page.