While this isn't teeeechnically the FR-1 Viper, its the same internals it was the same internals so hopefully this helps someone with the build.
As a US builder, I used some 1/8 inch rod for the pins, which equates to 3.25mm, so some sanding was required to ream it out to fit. I got the rod as Brazing rod from the local hardware store. Then I heated up the rod with a lighter and shoved to through to make the hole bigger. I did break through the plastic under the trigger rod as its quite then, so be careful. I alse used some nerf standard 2.6mm philips self tapping screws
For the wiring, I used some 18 gauge silicon wire (amazon link)
The battery I used was a turnigy nano tech 1300mah 2s battery pack (bundled with this charger to charge it and when ordered together gets free shipping!) and the motors were two honney bardger motors though Meishel 2.0 130 2s will work as well. and be sure that you get some kind of low voltage alarm so you don't kill your LiPos.
For the motors, it's going to be tricky. I suggest measuring out exactly the length of wire you are going to need from one motor to another when the motors are in the cage, should be about an inch. I wired the motors so they were wired straight across, which means that the positive wire needs to go into the red dot of one motor and the opposite terminal of the other. heres a good link on wiring a different cage. once the two were wired together I attached the red wire (which will go to the Omron switch) to one strip and the black wire (which will go to the XT60 connector) to the other.
The red wire is fed down through the hole and into the other side of the blaster and up into an Omron Switch. out from the "com" terminal. Then, another red (positive) wire into the "NC" terminal of the Omron switch. This is where the battery will plug into.
Then I hooked up the black (negative) wire to a female xt60 connector along with the red wire from the "NC" terminal on the switch. Make sure that the red is wired to the red of the battery and the same for black.
once the motors are in their housing (temporarily, just using the backplate to hold them in) plug in the battery and try the Omron switch to see which way the motors are going. don't worry about the motors, if they are wired backward they will just spin backward. see if they are spinning the right way to rip a dart out. if they aren't, unsolder the red and black wires from the terminals and flip them to make them spin the way you want.
now put the motors in the cage, and use the two motor mount screws to hold them in place. then attach the backplate again and now shove the FTW flywhees on to the motors. DONT POUND THEM ON. this is how I almost broke a motor.
For the springs I got a box of springs from the local hardware store and found an expansion and compression spring that worked for me.
I really hope this helps!