There are several places where you can get replacement gas blocks (with flip up front sights, detachable front sight, or weaver mounts).
But why would you want to remove the sight block from a barrel with pre-ban features, to use the same block w/o the features?
If you want to conver the barrel for post-ban use then use a dremel tool to cut off the bayonet lug, then some birch-casey touch up or some cold blueing to protect the exposed metal.
Most gunsmiths can/will do this for a very small fee and its MUCH easier to do than replace the front sight. Talk with Kurt's Kustom (see the Industry forums) he does this kind of work and can permanently replace the flash suppressor with a post-ban compliant muzzle brake that looks like the A2 flash suppressor(it must be a permanent instalation).