If the old timing gear has been replaced once already, you may find the original nut which retains it (the one which is "oval) has been "buggered up" by removal using a punch and hammer.
Punch and hammer works - but the damage it does prevents removal of the nut using the usually sold tool for removing it (below)
The "upset" caused by the punch prevents you from putting this tool on and doing what needs done.
So you may be forced into reaching between the radiator and the front of the engine to "punch" it off again - unless you can get a small grinder (Dremel?) in there to clean up the nut.
You may want to buy a new nut before you begin.
Joe K