I turned a Zebra F-701 into a poor man's Space Pen, and my favorite. You can follow all or parts of this and end up with a great cheap pen. I started with just replacing the ink cartridge and moved on from there.
Supplies: Zebra F-701, Zebra F-402, Fisher Space Pen pocket clip, Space Pen cartridge. You will also need pliers capable of gripping the little collar at the top of the pen. One of these will be metal, so be okay with scratching it or use some kind of rubber between. I also used some lubricant to smooth things out.
Remove the stock ink cartridge and keep the spring. Shove the Fisher cartridge from both directions through the tip of the pen to widen the little piece in there that holds the cartridge steady.
Remove the plastic collar from the top of the F-701. Pull out the clicker assembly, which should include the top button, the collar, and two additional pieces of black and white plastic. The pen barrel should be empty after this.
Remove the superior clicker assembly, including glorious metal collar, from the F-402 and screw it into the F-701. You may find that the clicker will squeak; the lube is good for this and a generous amount doesn't appear to interfere with the satisfying solid click.
Put a few drops of lube into the tip of the pen and move the Fisher cartridge obscenely through it; now it won't bind. I did this months ago and it's still fine.
The new pocket clip is because the stock one sucks (it has branding, and eventually breaks or bends). I tried for quite a while; it won't go on over the top of the pen. You have to place it against the barrel and push it into the barrel until it snaps on. Takes a good bit of force. This may result in superficial marking which can be buffed out.
I have other pens that many people would say are "nicer" (such as a TWSBI 580, or really any fountain pen over any ballpoint, in some people's eyes), but I use this one, even with the others within reach.
I let people use it, but not leave with it. It's not a big deal, just a pain in the ass to make a new one. I like to give them as gifts so eventually it won't be an issue anyway. :D