3020 diesel was a very good tractor. Other than checking normal items such as condition of engine oil, transmission oil and coolant for contamination, etc, the only thing that comes to mind is the injection pump. Some of the later 3020's had a CB Roosamaster pump. Many parts for those pumps are not available anymore and could require an expensive upgrade to the more common JDB Roosamaster pump that will require an adapter bracket and new fuel lines to install. That can get expensive. If powershift, be sure tractor pulls strong in all gears. Also check the pto runs and stops when it is supposed to. If synchro range, also make sure all gears can be gotten into and shift linkage is not worn out.
