Yeah, KJWs aren't bad. In the heirarchy of M9s (and most other pistols) commonly available in Canada, I'd rank it third best, behind TM and KWA. My very first KJW M9 is still running beautifully after a few seasons of relatively heavy use.
Replacing parts in the G17 would make it more useable, but you'd basically be replacing the parts with TM parts - or parts made for a TM. The WE should be compatible, and if it was built correctly, it would be compatible. But things just might not fit.
You can take a look at this thread, to see what you can upgrade in a TM G17:
http://www.airsoftcanada.com/showthread.php?t=140496
Theoretically, everything should fit fine. Maybe do some research on each individual part first, before purchasing.
I still don't think that it's worth the time, money, and effort. But if you want a project, and don't set particularly high expectations for the gun, then go for it.
Just be aware that WE pistol parts suck, and that you may end up with just a TM pistol in a WE lower receiver with a WE slide, by the end of the project. :P