Simple API mapper

Is there a node which is a simple JSON mapper (i.e node receives JSON and reformats it to different JSON by providing mapping of element names from->to and maybe format conversion, then calls an API with the resulting mapped content?

Sounds for me like the “Rename Keys” Node could help you with the mapping:
https://n8n.io/nodes/n8n-nodes-base.renameKeys

If you want to change the format, a Set-Node would probably be best. You can simply set the key-name (can be the same as it is right now) and then reference the current value with an expression. In that expression you can then use the JavaScript functions parseInt, parseFloat or .toString.

Hope that helps!