Fuel gauge requires fuel level sender (located in fuel tank). May already be there. Look in fuel tank; look for arm with float on it. Sender mechanism actually has 2 senders, one to low level light in speedo/tach cluster, and float mechanism which drives fuel gauge. Check first to see if low level light works. If u need float mechanism and sender, they are occasionally seen used on ebay. Bear in mind that if fuel pump/sender and associated parts are removed from fuel-tight connection, new gasket and probably fuel pump mounting damper (correct name escapes me right now-old guy disorder) are probably needed. Connections are located right side, coming from bottom side of tank. Search old postings for pinout colors.
May be better to leave well enough alone and follow advice of one who uses odometer as fuel gauge. Ride safely.