State laws will dictate bare minimum. You need a headlight in all states. Some states don't require turnsignals.
Instrument clusters you don't need if you are going for the clean look. I'd stick with some sort of speedometer though just to avoid tickets though.
If you are eliminating the fuel gauge (with the insturment panel) then of course you will want to know the range of your bike so a speedo/trip in handy.
Patriot Guard Rider
2000 VN750 Sere (Serendipity)
1990 GL1500 (Ole Blue)
1986 VN750 EVie (project bike, heavy custom)
VROC # 11628 / 25000-H
ARVROC # 12 Coordinator and Crowd Control
OKVROC # 18 (H)
TNVROC # 45 (H)