Install a new cylinder and re-jet the carb back to stock. If the ported cylinder has been bored you will need new piston and rings also. I would re-jet the carb first and see if that change will suffice in the fuel economy area. Changing out the cylinder etc., is costly. Check the front and rear sprockets. If they are not the stock sprockets, ( 14 tooth and 48 tooth ), replace with stock. Please rate my answer.