If your bulb stopped working you can send it in to Streamlight for service, or take it to an Authorized Service Center. Check out the link below for more info directly from Streamlight.
https://www.streamlight.com/resources/learning/frequently-asked-questions
Streamlight does not provide the mounting length of their TLR-1 Game Spotter Green LED Gun Light. I would recommend contacting them directly for more information.
The Streamlight TLR-1 Game Spotter Green LED Gun Light for Game Tracking mounts to picatinny rail. So if you had a section of picatinny rail on your scope mount then it could be mounted to it.
You can purchase the Streamlight TLR-1 Game Spotter Green LED Gun Light with or without a remote switch. The model that includes a remote switch is MPN:69228
The Game Spotter Green Light is for preservation of night vision. If you're looking for this in a white light version, check out the TLR-1 HPL.
https://www.opticsplanet.com/streamlight-tlr-1s-hp-strobing-rail-mounted-tactical-flashlight.html
This uses CR123A batteries - there are both disposable and rechargeable versions available.
You shouldn't have any problems mounting this on a Ruger PC9.
You may be able to be adventurous and try to replace the head with the TLR-1 HPL Facecap. No guarantees this would work, but it might be an inexpensive option for switching over.
https://www.opticsplanet.com/streamlight-facecap-tlr-1-hpl.html
Yes, it will if mounted properly.