I haven't seen a volt meter that would fit the hole but I'm sure they are available somewhere.
In the interest of returning the look back towards the past, here's a thought:
Replace the fuel/engine dial with a suitable volt meter that looks like the old ampmeter. I'd say use the old RE ampmeter but adding the needed wiring branch from the battery to the ampmeter and from the ampmeter to the ignition switch might be beyond some peoples ability. A volt meter could be attached to the headlight's gray "running light" lead like I did with my LED voltmeter. Then, just a simple ground wire connected from the new volt meter to any part of the frame would get it operating when the lights are turned on.
OK, so now the meter is in place but what to do with the other lights?
IMO, the fuel level light is worthless. Even if it worked on my bike (which it doesn't) I never liked it winking on and off when I still had a half tank of fuel so, it could either have its bulb removed or just let it dangle inside the casquette.
The engine light is important. Not only does it turn off after a successful ECU boot but if the bike develops sensor problems it's multiple blinks will tell you what's failed. Where to put it?
How about plugging it into the High Beam headlight socket that's located in the speedometer? I'm sure the flashing blue light wouldn't confuse anyone unless they were addicted to WalMart and couldn't resist the flashing blue light specials.
I seldom use my high beam except for flashing oncoming cars at major intersections but even if I did use it often, it's easy to just flick the left hand high beam switch a time or two to know which light is being used.
Anyway, it was just a thought.