3) a piece of modern electronics, a battery to power it
The only "modern" piece of electronics I put in is the 60mm GPS speedometer available just about everywhere. It fits neatly into a Vespa Grande headlight housing. No magnets, no extra wires: pop the switch and it starts looking for a signal. Mine is a Latnex brand and it works. I'm sure it's a rebranded item and is available everywhere under different brand names.
The headlight is the Larsen brand out of the nearby Fergus Falls, MN. It has Hi/Low settings and is blindingly bright at either one.
The LED tail light has the brake light function. The China master cylinders with built in brake switches work.