The 413 and other Stylist series machines were designed to be able to replace the gears. And it is a fairly straight forward job. The 247 that CMARAS has and others in that series were throw away machines. The bottom of the gear sits in a well and the new gear can not be slipped into place without removing the main shaft.
If you have a metal gear that matches the old one in all measurements you can interchange them. Do make sure you replace pairs of gears as a 'new' metal one would not work well (in the long run) against the remaining old plastic one.