I'm pretty sure the Valk speedometer is cable driven like the Max. If the rim/tire profiles are similar I don't see why it wouldn't work. Most speedo's use a pretty similar cable drive, I've interchanged cables between various brands of bikes and even from snowmobiles.
Not really sure about the tach....the vmax one just reads the square wave of one coil switching on and off, and how the vmax ignition is, 1 pulse=1 revolution of the crank. The stock tach reads off the rear left cylinder if I remember correctly.
You'd have to do a little research on the Valkyrie's ignition. The Vmax behaves like a 2 cylinder engine in that regard, but using diodes you can tie multiple coil signals together to effectively double the number of pulses(a trick that works for tachs that only go down to a 4 cylinder mode, otherwise it reads half of actual rpm). Since the Valk is carbureted, chances are it uses a trigger coil like the vmax, not a crank sensor as EFI bikes do, so it's probably a waste fire as well. In which case it would "behave" like a 3 cylinder, and wouldn't work (or be reasonably adapted to work) on the Max without some complex electronics to modify the signal.
Using a gen2 Magna's tach would probably be easier since it was also a v4.
