580CK older model replacing worn input gear

[Higherground]

I have a 1967 Case 580CK loader/backhoe.

I need to change out my forward gears, they are worn and will not engage when put into forward. Reverse works fine. Attached are a couple of pictures showing the worn gears.
The left picture is of the forward gears, and the second is of the reverse (no wear).
I need some guidance as to replacing these gears.

Thanks,
Harold