What RJT said.
If you Google search "lionel 671 exploded view", you will find the exploded view showing where the thrust washers go. If memory serves, there are at least 2 and they could be different thicknesses. You may have to reposition things more than once.
If that doesn't work out, try taking a video of what you are seeing and post it here. I too am confused about "I put my finger on the gear and push forward, the motor starts". I am not sure where you are talking about. These old machines are so simple, it's not likely this is unfixable; possible but not likely.
Tony V