The large silver hose is a hot air duct from the exhaust manifold to the air filter. It is suppose to run across the bottom of the radiator to get to the air filter.

Yes your '81 is probably a k-jet like my '81, a USA California or Federal version. Virtually all gasoline powered cars have some kind of pre-heat to help keep the fuel vaporized and prevent it from condensing on the metal walls inside a cold engine. It thereby increases combustion efficiency and reduces air pollution during the warm up cycle.

The aluminum ducting should be available at any auto parts store. You should check that the thermostat in the air filter box is open when the engine is warmed up. Pre-heated air to a warm engine can reduce mileage and possibly cause pining. Just remove the air filter element and at the bottom of the filter box you will see a pivoting cap at the air duct connection. It should be in the open position when the engine is warm.

It is the metal tube from the back of the water pump to heater hose on the drivers side by the head going into the firewall. Junk yard or Volvo would be the only place that has it.
Dan