>>1536468shit tier old road bikes used to have 'claw derailers'
and you can get 'derailer claw adaptor' to attach a decent derailer to them
so i'd look for that instead of a track bike specific part. I don't see why it wouldn't work.
You also have to spread the frame because it'll be 120mm and so i'd try get an oldschool 7spd 126mm hub if possible but i guess you could just cram a 130mm newschool hub in there
then you need cable stops too.