I had a 1999 M600 that was a fun little bike, and a very simple beast - it'd probably be pretty cheap to find a Monster ignition & seat lock (which are keyed alike) that would be a direct plug & play, no red key involved.
Since the 600 was a lot more common on your side of the Atlantic, you might even get lucky and be able to lay hands on one of the cylinder/piston kits that bumped up the displacement to around 750 (if I remember right) and the HP & Torque to well above stock M750 levels.
Good luck - that was a fun bike.
