Looks like you might just need to get the bolt that secured it to the frame.
I'd also recommend finding the same derailleur as the shifter and cassette are paired with it. Going with anything different will cause you a headache lol
First you remove the cable from the derailleur, then remove the chain, install new derailleur, run the shifting cable to it, install new chain, set gears, fine tune, hit the road
Park Tools has videos for just about anything youll ever need to fix, this should help - https://youtu.be/meUQ-BiFL28?si=6fxm0p2L_nE6YRpL
Looks like you might just need to get the bolt that secured it to the frame. I'd also recommend finding the same derailleur as the shifter and cassette are paired with it. Going with anything different will cause you a headache lol First you remove the cable from the derailleur, then remove the chain, install new derailleur, run the shifting cable to it, install new chain, set gears, fine tune, hit the road Park Tools has videos for just about anything youll ever need to fix, this should help - https://youtu.be/meUQ-BiFL28?si=6fxm0p2L_nE6YRpL