Someone asked about goggles over prescription glasses...
Our host sells a nice pair which I use regularly with my HCI retro helmet (expensive but they are the real deal!). For cold I wear a lycra balaclava with a brushed lining (Schampa). Scrumptious even with the retro helmet, at least down to mid 40's as it's been here at night. Fits better than the fiberpile-lined nylon bandanna I bought last year.
Shiny black snowmobile bibs with full leg zips over jeans (with wool or polypro tights for the really cold). I haven't been able to fit into them for 18 years but now they fit just fine. (The absolutely fantastic no draft riding jacket that I bought with them was stolen from my garage the very next spring. :O( Hmm, that was the last garage I had, too :O(
My 28 year old Belstaff Trialmaster is &^!*%$ in winter. I road from Elizabeth City, NC to Boston, MA in late Sept 1978 and when I took my Belstaff off I stood it in the corner like a mannequin. Unless you buy one size larger than than your normal you can't fit much of anything into the tight fitting arms. And in the summer you swelter in it. I love tradition, but that jacket is a spring and early fall jacket only. The pants work well on long road trips in mild weather but are not slippery lined nor full zipped and are a bear to don at roadside (tuck your pants into your socks and get good at balancing on one foot).
My Pakistani made ($149) black leather jacket by River Road has a zip-out liner and is draft free and all around great. Loads of room for merino wool shirts and great miracle fabric zip turtle necks. Multiple thin layers works for me.
Red Wing engineer boots (very tight at first but they will break in believe me!) over Smartwool socks.
Gloves are the bummer. I have a pair of mittens but they feel clumsy and without using the "hot pocket" chemical packs they are no warmer than my gloves. Might try an Aerostich overmitt or the Fieldsheer mitts someone mentioned here.
I used to ride the round trip from Boston to Youngstown, NY on frequent weekends, three seasons of the year. An old hand-me-down red quilted nylon snowmobile suit was my favorite then, complete with man-made fur collar. But then I was younger and had more mo-jo to burn.
In winter the windshield goes on the Enfield, f'sure.
-Jesse, always stop for pie and coffee, it's part of the trip, man.