Verify that the internal surfaces of the z_motor_mount are smooth and flat. Clean up if necessary using a file or small scraper (you can use a steel rule if you don't have a small scraper).

Verify that the surfaces that will be in contact with the extrusions are smooth and flat.

Take one of the Z smooth rods, which you disassembled from your Prusa, and test fit it into the Z motor mount hole.

If it is too hard to insert the smooth rods you can use an 8mm drill or reamer to increase the hole diameter. Don't pass the drill / reamer through the full depth of the hole, only half of it is generally enough and will ensure you keep a tight fit. Remember, don't use a power tool, use your hands :) !