Bob, essentially the size of a muffler has to be proportional to the capacity of the engine. If it is not you have to settle for either less sound reduction, or more back pressure (which reduces power, and may even burn the exhaust valve under sustained full power). A GX160 is 160 cc, and from memory the G65 is about 400 (don't have my real computer running, I'm using the backup for obscure reasons). Also the exhaust port size tends to be in line with capacity, so you'd probably have to modify the GX160 muffler to get it to match the G65's port.
