Hello, @Peter Simpson (UK) - Thanks for reaching out!
If the connector is confirmed to have not returning all of the fields from SNow despite having specified them in the query which is its only available means for such functionality, it is more of a supportability issue on the connector side, and if that is indeed the case, I recommend submitting a feature request on the Feedback Portal for the Product team to consider & prioritize the matter in the future roadmap. In the meantime, you'd have to construct your own solution using either the HTTP Connector or building your own custom connector which may not be as substantial of an undertaking as it might sound.
On the other hand, if the connector is returning all of the fields and more data such that it is more of a matter of filtering out the ones you actually need in the subsequent actions of your Logic Apps workflow, you'll want to take a look at the Data Operations features in Logic Apps: Perform data operations in Azure Logic Apps
Depending on your exact requirements, you can transform the data fields by using operations such as Compose, Parse JSON, etc. Let me know if you need any assistance on this part. If you provide an example data set that you are already receiving from the connector, and an ideal data set that you'd like it to transform to, then I'd be able to provide you with a more specific example for accomplishing the transformation.
I hope this helps, let me know if you have any further questions ;)