Once I did some keys for my Casio PV-16 using sketchup to model. Fine adjustment were made with file and sanding. Not perfect but close enough.
@Danjovic
Nice work. For the keys I'm working on right now, I plan to match-color the old keys. Thankfully, all the missing keys in my computers are dark-gray ones. I have a resign with a matching degree of gray. As for the labels, I put carvings on keys' surfaces and plan to fill them with yellowish resign.
@mohai
I'm using SketchUp to do all of my CAD work, such as 3D printables, landscape layouts, furniture, etc. I know it's not the best, but it's easy and I have got used to it (been using it since it was a Google product).
Ok, the last prototype seems to be good. I uploaded the right shit key design to Thingiverse
The curvature in the front side is due to the hot environment I operate the printer in (I can't bear the resign smell so I keep the printer in a stockroom!). When I print the final key, I'll move the printer to a room with an AC for an accurate print out!
Thank you @Nprod. For prototypes, I use Elego water washable resin (I got it correct this time 
). It is less smelly than the regular stuff, but it still smells never the less. I'll try to get the one you recommended and see.
@Alexey and others who own AX-200 or YIS-503II,
I'm designing the spacebar key and have tried my best to imagine how the original hooks were be. As you can see in the pictures below, I put the hooks' openings toward the user, i.e. facing away from the rest of the keys. I'm not sure if this the correct orientation. I'd appreciate if someone who owns this computer to take off the space key and upload a picture of its fixture.
Those holders are facing forward and the ends of the metal clamp go into these holders to prevent the space key from tilting when a user pushes on its sides. Here are some pics.
Note that the gap for the clamp is quite narrow, about 1mm and it has a small bump on the inner side to prevent the clamp from falling out.