Even easier, & probablly cheaper.
You will need: 1/8" rod, 1/4 rod or bolt & a piece of reasonablly hard material.
Drill a 1/4" hole in the hardest material you have around. Corian works well, a fairly hard wood like maple will also work for a while.
You need a very long nail, or rod roughly 1/8" diameter to poke down through the transmission to the inside of the nib. Using a hammer on the rod sticking through the transmission and resting on the inside of the nib, tap the nib out.
Now, insert the transmission into the 1/4 hole and put the bolt into the other end of the pen/tube (where the nib used to be) and press, not hammer or tap the trans. out of the tube and into the 1/4 hole. You should be able to keep from doing major damage to the lower barrel wood as it is held by the blank while the trans. goes into the hole.