You didn't say it had a carbon fork. If it had the original steel fork then threaded/quill stem would suit it better imo. But since it has been altered (why?) with a modern carbon fork and perhaps other modern parts, theadless won't matter as much aesthetically or period correctness-ly. But finding a suitably sized quill stem will be your cheapest route, perhaps you can trade? Nitto pearl stems are nice, not Ti but high quality alloy.