How can I replace the 340-020 movement in my 74 mantel clock?

Just check on the web for clock parts as there are several suppliers that you can get movements from; try R&M Imports. Remove the hands, remove the wood screws that hold the movement to the front of the case (accessible from the back) and pull the movement out. Revers the process for putting the new movement in.
By the way, there are no "unrepairable mechanical movements" but they can be more expensive to repair than replace.

Stevens Clocks
February 2013