If you have already tried the obvious, contacting Walther's, then the next oprion would be to try one of the firing kit companies. If this kit is suppose to function "just like the real thing" without actually discharging a projectile, then I would guess that it is going to be assembled somewhat close to the real thing.
Also, if it is as advertised, you might want to invest in a spring vice. This is a specialised tool just for installing flat, main springs on firearms. You should be able to find one of these at either www.trackofthewolf.com or www.brownells.com.
One manufacturer of kit rifles is www.dixiegunworks.com.
I hope this will be of some help.