NT would have to "catch you at it" and remember the NT are hard pushed to fix gates and consolidate monuments let alone aggressively patrol against freelance interpretation guides. If you had already reached a private arrangement with a client/ clients to "show them the stones" they cannot do a damned thing to stop you.
Do it.
Halfway through your tour, pull out a pocketful of scrapers and show them to the visitors. Ask only a fiver for each one. And point out that the NT shop sells "modern" ones for nine quid. They'll go like hot cakes.
I remember arriving on North Uist on a Sunday morning many years ago. Everything was shut, closed and barred by the "Wee Frees". Nothing to do and nowhere to go. One enterprising guy had slung some tartan rugs in his clapped out minibus and was running sightseeing tours round North Uist and Benbecula etc. Then he'd return you for the afternoon ferry. He only charged a couple of quid. It was brilliant. He chatted and pointed out stuff, stopped for photos for people etc. He even handed round a beautiful neolithic axe (perfect condition) which he'd found. You still remember those kind of "tours"nearly half a lifetime later.