Threaded 9/8 Quill stuff is rare as hen's teeth, [X mart bike?]
if there is a threaded headset and it is a 9/8 size , the path of least resistance
is a quill to threadless adapter plug, then you can shop for abundant choices in threadless stems.

the adapters are made with 7/8 and 1" quill OD's thats the ID of the fork.