Take the screw to the hardware store, to compare it to the existing screws for diameter. It should be either a #8, or # 10 screw. Next, ask them for help in determining the thread count. It may be metric, but Most of what is coming over here from Italy is using American Threads. This will help you learn what a thread gauge is, so you can buy your own.
Now, go home, and go to the Links section on the Index page, under Member Resources, and find the list of suppliers. Dixie Gun works, Track of the wolf, Cains, and many more suppliers can supply you a replacement screw to get that import back in business. All they need to know is which lock bolt you want, and the maker of the gun.
IMHO, the only way you break screws, is by trying to HORSE a screw in or out. Spend less time at that "neat little bar", or don't work on your gun when you come back from it. Guns and booze really are not a good mix. I would never discourage anyone from drinking, but I have so many stories about guns being broken when someone comes home after the bars close and decides he has to get his gear together for a Saturday Morning shoot!-- well, leave it at " guns and booze don't mix!"
We have a few members here from the Houston area. Perhaps some of them will come on this tread and contact you with offers of help.
You have a fine Texas Muzzleloading Rifle Association, which you should contact and join. Several of its members are active here, but live in other parts of the state. Davey, and Roy, and John Hinnant, and Rabbit03 come to mind. All would be great resources to help you with that build, but you would have to expect to drive over to their shops.
Don't hesitate to call or contact local gunshops, to locate other gunmakers. This is a small community, and generally the word gets around as to who the local builders are. And, many general gunsmiths can also help you with some of the problems you will find as you work on the gun.
Don't forget to click on the Gunbuilding Tutorial on this thread. Its located at the top of the page for " The Gun Buider's Bench." Done by Mike Brooks, a terrfic gun builder and member here, the photos do a fine job of taking you through all the processes of aligning and fitting parts to the stock.
There are also tremendous Gun building videos available from
AmercanPioneerVideo.com, showing Frank and Hershel House building guns from scratch.