Originally Posted by TenGrainBread

Nice find! Old Jamis MTBs are pretty sweet. I believe these were first TIG welded and then fillet brazed over the TIG weld for aesthetic purposes.

I wouldn't assume that your headset is 1". It might be, but 1 1/8" threaded headsets were also popular around that time. Measure the quill (insertion) diameter with some calipers. Once you know the headset size for sure, you can choose a stem and bar combo. As far as the stem goes, for a dirt drop mountain bike you want a tall stem with not much reach (as the drop bars will stretch you out).

For 1" threaded stems, I would check out the Nitto Dirt Drop stem. You can also use a tall Nitto Technomic stem. These are 26.0mm or 25.4mm clamp only, so you will need handlebars that fit this. Nitto's Dirt Drop bar comes in a 26.0. You can also check out the Rene Herse Randonneur bend bar, in a 25.4, also made by Nitto. If you need a 31.8 clamp diameter stem but don't want to use an adapter, Velo Orange makes a tall 1" quill stem with a 31.8 removable faceplate. It might not be tall enough, though. Another option is getting a 1" threaded to threadless stem adapter and then using a threadless 1 1/8" stem, like the Velo Orange Cigne stem or the Crust Bikes LD stem, as you mentioned.

For 1 1/8" threaded stems, your options are more limited. Most vintage stems of this dimension were very long, meant for flat bars. Pairing these long stems with drop bars makes fitting the bike very difficult. Sunlite makes a 1 1/8" aluminum quill stem that I have used before. It has a faceplate for a 26.0 handlebar. While heavy, the finish is decent on it and it gets the job done. The other option is to get a replacement 1 1/8" threadless fork and then run a threadless stem. This widens your options but costs more, and of course you don't get to use the Jamis fork.