So NOT period appropriate!!!
Bike computers are great, There are several out there that will read up to moto speeds and highly tunable to get very accurate speed as well. No light and definitely does not fit the character of the bike IMO though. I'd love to see one that offers a 12V backlight for motos.
If you're ok with the hi tech look go with the first option that combines speedo, tach, lights, and I think a few other things into one clean unit. I've seen similar units put into other bikes and I think someone installed one on an RE here on the forum recently. Nice units. I've also seen tiny units that do the same that are maybe an inch tall and a few inches wide. They're nice if you want to eliminate a big round speedo. So you could put in a nice big analog tach and a smal hi tech LCD bar computer multi-display.
Does anything fit in the casquette including the stock headlight and speedo?
If you went with a small tach you'd need a filler plate to make up the difference. Might be able to sneak in some lights and other toys. I would bet you could get another electronic tach that fits. You could probably find one that's shorter top to bottom than the stock speedo and so easier to fit, though I haven't had much luck finding tachs that work for single cylinder engines. Most are for 2 or more cylinders.
There are lots of little tachs and I think they'd fit in the idiot light spot but they're either for twin cylinder engines or mechanical. Harley stuff near as I can tell and won't work with our engines
Scott