Retrieve Asset Metadata from metadata-hash

How can I retrieve ASA meta data (in original .json format) from its metadata-hash attribute?

It’s impossible in most cases.

Hash functions are meant to be difficult to invert if you don’t know what was hashed.
See Cryptographic hash function - Wikipedia

If you know the ASA metadata, then you can check easily that it matches the hash.
But you can’t do the reverse operation.

I believe your best bet is to look at the URL (if used) specified at creation and grasp whatever metadata you can get from that.