Alpinestars used 1 1/4” stems so your options will be somewhat limited.
110mm is probably about right by retro bike standards
120mm and 130mm was deffinetly a thing and so was 0 degree rise.
Catalogue spec shows AS Mlite Cro Mo as the stock stem. (With cable guide)
Try 100mm see if it suits you better, anything shorter would look out of place.
Answer ATAC quill stem would be a nice touch!