I did this on mine recently and its actually not as hard as it seems to achieve what you're after. I wanted the same! Full power for now but the ability to put it back easily if required (insurance, licence restrictions, full resale value for a LAMS bike etc.).
The plate has two screws holding it in place at the top and bottom. They're designed so you can't undo them with the screw heads being smooth and cone shaped with a small flat peak.
What you need to achieve is removing the top screw which allows you to rotate the plate clockwise just enough so the throttle can turn further.
All we did (a mate helped me out) was drill out the top screw, removed it and then replaced it with a normal phillips head screw that matched the thread. Meaning at any point I can simply use a screwdriver to loosen the top screw a little, rotate the plate anti-clockwise back into place and tighten the screw. Voila, restricted again.
The hardest part is getting the stock screw out. We drilled through the centre of it at first to make a hole (we strategically placed an old rag beneath it so metal shavings didn't end up throughout the bike) and then used a special drill bit to unscrew it. I can't remember what it's called (maybe someone can help with that?) but basically the thread on the drill bit is backwards so that when you are using the drill in reverse (spin anti-clockwise to unscrew) the drill bit thread is actually driving in further causing a gripping effect which in turn then unscrews the stock screw. My theory on how that works might be a little off but it exists for sure and it works perfectly. It's just a drill bit too so you can pick one up from a hardware store for cheap.
And all of that was done between the metal tubing of the naked frame, harm free (be careful of course), without the need to remove/dismantle anything.
Hope that helps, I know i'm a little late to the party
Happy riding! I absolutely love my 659, couldn't have picked a better first bike!
