The whole point of the K5 with its Jager Tilt Block is that this controls all the headspace etc. the actual hinge pin is just for opening and the locking lugs to keep it closed.
By loose is it just nice and easy to open or is it all rattling about.
If its the latter the a larger hinge pin / hook made smaller - very much a job for a really competent gunsmith, and one who understands the break action kipplaufs. Alan Rhone or whomever imports the Merkel should point you in the right direction.