OK, so know I know a little more about what I need and what is available, I have learned that there are two options available to me: there are quite a number of replacement quill stems including some angle-adjustable ones and also there are quill converter posts that would enable me to use a "modern" stem and handle bar. I just need to find out the diameter of the quill stem, it appears that it is likely 25.4mm but I'll pull it off and check it out.