You could, but the rebuild would require a new casing, a small coil, and it would still be too bulky to carry around.
Before you have a usable pinpointer you would have spend more money than if you got a propointer in the first place.
If your interest is about a great winter project, and the joy of building something, then look at the harbour freight pinpointer, and do the mods you can find around this site. Somebody extended the wand and attached the pinpointers body to the detector, like a sunray probe. I liked that. There's some info in this section describing how to make the cheap harbour freight pinpointer very nice and powerfull!
GL
/Steffen