Bill,
Remove the bonnet and place on a flat surface. If working alone disconnect the stays from the rear of the bonnet leaves, undo the two hinge fixing plates then fold the two halves together to one side with a piece of thick cloth between the leaves to prevent damage. You can then lift it off single handedly.
There are (should be?) two end stops. one each end of the hinge, to centralise and keep the bonnet halves correctly positioned. If your bonnet currently fits well make a note of their positions then remove them. With the bonnet opened out on a flat surface withdraw the hinge either way. If it is bent or badly corroded it may stick so you may need extra hands..
Grease the new one before fitting it and replace the end stops and when putting it back I suggest cloth over the top of the grill and scuttle so you can relocate it without risking damage.
B……….