You should probably post this in message in General Support forum https://www.nopcommerce.com/boards/fg/5/general.aspx
And CCAvenue Plugin from nopCommerce team is available on https://www.nopcommerce.com/p/962/ccavenue-payment-module-nopcommerce-team.aspx this plugin is FREE and has source code available with it. using source code you should be able to make same code work for old version 2.6 of nopCommerce.