There is no issue with restoring a sporter back to GI configuration. In some cases it can be virtually undetectable.
The issue is originality, and whether or not someone is paying for that. And I don't mean just all originally "correct" parts & markings. A dedicated collector makes a distinction between a piece that is "in original configuration" and one in "original condition".
An example would be a GI 1911, maintained by the Army with 1911A1 parts.
Or a Garand, with a mix of Winchester & other makers parts. Totally authentic, and correct, but not "original", the way they left the factory.
Collectors are funny about some things, to some of them, a gun that while totally correct in parts and appearance can be two different level of desirability. One that has never been changed can be on one level, and one that was changed and restored, on another level. This can make a difference in the price to a collector.
Claiming the gun has never been changed (and priced accordingly) when it has been changed, and restored is fraud. Claiming it has been restored (and putting any price you want, on it) is not.