Is there a script that can serve as a p2p to http converter?

Question

JavaScript

That is, by specifying a torrent file, or a magnet link and give out an http stream to a file or files

Answer 1

Standalone JS script - no.
This should be a full-fledged service that has a torrent client on the server and downloads files via p2p, and then sends the file via http.

Answer 2

On the node - easily
https://github.com/webtorrent/bittorrent-tracker
And in the P2P browser you can arrange via webRTC , and if you find (or write) the appropriate torrent tracker, then you can send data via HTTP somewhere.
https://webtorrent.io/ not?