That adjustment can be used to turn down the power as well as turn it up. I use it if I use a heavier pellet than normal ( i.e in windy conditions I use a heavier pellet to normal summer days).
I can't see yopu getting in trouble just cos your stock has been cut away, obviously though its up to you to make sure you are legal. Cheap chrono's start around £40 well worth the money I'd say.
There is a MK1 stock on Ebay at the moment.