I don't think that's as easy as it may seem.
There's a thread or two on here where someone detailed his adventure of replacing the ODO of one with another. It was not a small undertaking.
If your speedo part still works, it's most likely your gears in the reset trip odometer button section.
I researched this issue years ago and I "pounded" on my reset button while rolling down the road and all at once my odo started working again. I have since put black tape around my reset...