Are you sure the muzzle is actually threaded. Some appear to be, but are not. Also on most NHM 91 the pin to hold the break locked in place is not spring loaded, it is fixed in place and requires some smithing effort to get out.
My NHM91 is currently at Rifle Dynamics being shortened. They make awesome carbines with the heavier barrel and quality Norinco receiver. Very accurate for an AK. For me the shorter gun is more versatile. IF the muzzle isn't threaded you can have a smith thread it with the treading of your choice and put what you want on there. If it is threaded, you can simply replace it with any Chinese AK break because the threaded ones were milled and narrowed to fit the standard break. Likely the front sight base is a standard size as well and the barrel was likely milled out to to fit it on.