You might want to consider why those particular parts are missing - - - - -(Hint - - -they 're e$$$pen$ive!) scarce, also! Big truck parts have a tendency to make the old checkbook scream in pain! OLD big truck stuff is even more in that category, because most of them got worn out and sent to scrap. The new and/or reconditioned parts supply dried up due to lack of demand, and unless you find one parked in some farmer's shed, the used parts supply isn't plentiful either.
Jerry