Picture of Powerline 880 Air Rifle Bullpup Conversion
Bullpups with the trigger group behind the trigger, using a trigger rod, are famous for having bad trigger pulls. This mod actually improved the trigger pull (lighter, smoother, adjustable for overtravel), while coping with the fact that the gun has an underlever right where you want to put the pistol grip.

Accuracy is approximately as good s it was pre-mod (about 2 MOA @ 20 yards), though it's been in storage in a humid basement for a few years after the old stock broke and seems to have developed a pickyness WRT ammo

Overall, a build using inexpensive materials with a fairly low tool requirement.
All replacement components are fabricated from poplar, steel, or PVC and held together with epoxy or screws... plus a single rare earth magnet, and a tiny bit of aluminum and hot glue for the trigger. (Total cost for materials used was about 10$.)
You'll need a vice, a hacksaw, a drill, a set of files for wood, a set of files for metal (or a Dremel and a bench grinder), and some way to make boards of a specific thickness. A laser sight would be a real convenience, and a band-saw would make some steps a lot faster.

Because of the number of details in the project, I won't be doing a step-by-step for the entire project, just for a couple tricky bits. I will try to provide good pictures and descriptions to let you know what design-type stuff I came up with.

Anyways, onwards...
...to prime you, due to geometric dependencies, I recommend building the parts in the following order:
  1. Remove old stock. (Screws? Unscrews! Iron sights? Hacksaw!)
  2. Make forearm, except for scope-riser hole. (such that it fits in the receiver)
  3. Make Pistol-grip (such that it doesn't bang into forearm)
  4. Make Trigger group (such that it works inside the pistol-grip)
  5. Make Butt-stock (such that it gives you the right length-of-pull to the trigger)
  6. Make Scope Riser (such that it puts the scope in the right position)
  7. Finish forearm (with a hole that allows it to fit over the scope riser without touching it)