to find the threa it has now take some blue tac and a strong piece of wood . Put the blue tac on the wood and after running a little fine oil into the thread apply it into the thread then withdraw with the blue tac ( hopefully ) still attached to the wood. You can now measure the thread using a thread gauge. Measure the diameter of the hole and that should give you a fair idea .
If the lathe is 1’ by 8tpi you can buy a ready made chuck back plate from RDG tools in 4 or 5’ diameter , it’s a common size . You then need to replace the one that’s on the chuck , assuming it’s removable ( most are )
If there’s enough metal ( ie it’s definitely smaller than 1’ ) you could have it bored out and re threaded if you know any friendly model engineering guys
