I have a few components of drones like some old FCs and old ESCs and I don't want them to go to waste. Is there any way to configure a drone that I built with those parts, as I don't think the FCs are capable of running BetaFlight. (are there any really simplistic, light things that the FC can run instead of BetaFlight?)
You can always run an older version of betaflight. Also, if you have older F3 controllers you may also like to like to look at https://github.com/spatzengr/betaflight/releases which are maintained by UAV Tech.
I have used these on some older F3 boards and it breathes new life into these old boards.
Librepilot may be able to do what you want. I had it running an old CC3D board recently until I burned it into a parking lot at high speed. I prefer Betaflight so far, but I haven't really explored either all that much yet.
You'd need to provide more info on what "old FC" is, and same for ESCS.
For the FCs - are they running an STM32 (F0, F1, F3, F4, F7) processor? If so the cleanflight/betaflight path is what you want
Is it the KK2 board? If so - you want kapteinkeuk (or steveis for newer stuff) firmware.
The KK01 boards (analog gyros) run kapteinkeuk firmware
Most anything else with an atmega style microcontroller will run multiwii if it's configured properly.
As for the ESCs - they likely either run simonk (again atmega) or blheli (silabs, I forget the manufacturer of the BB02, and stm32).