It would be better if you could post pictures of the rifle. The Jukar locks and breeches varied over time.
I'm 99.9% sure the drum is threaded since you haven't launched it when the gun was fired. Whether it is just a simple drum or the patent drum that goes into the breechplug is another question. Early ones had a simple drum. In the mid 70s the patent drum was coming into use but I think that was done by Andresa.
As far as the lock goes, I'm pretty sure you will probably have to inlet another lock in. If it has the Maslin style lock that Traditions and CVA use then you can get a drop in lock. The flintlocks on these rifles were pretty cheap and mostly were very bad. There are some that work fine but it is a crap shoot if you get one. You may want to get an L&R RPL in that case.