I found a solution by fetching the paid article html and replacing the content section with the response of the Admin API:
https://github.com/F-Node-Karlsruhe/ghost-iota-pay/blob/main/ghost_api.py
To OP: For having a html field in the response the ?formats=html part in the request url is essential.
api_url = '%s/ghost/api/v3/admin/posts/slug/%s/?formats=html' % (URL, slug)