So long as you NEVER attach it yes... The same way I can own a 10" barrel and upper receiver but I need a Federal Tax Stamp to attach it to a lower receiver and make a SBR. Rules be rules, whether they make any sense or not.
Polyethylene is light. Having a removable fuel tank likely saves scant weight, but it's really convenient. But while there is no specific list of equipment that "must" be carried on a Part 103 "not-an-aircraft" there are things that "bust" it out of Part 103. Fuel capacity over 5 gallons is one of those. I know somebody who has two five gallon "football" tanks hung from the jury struts on a parasol. But there is only one set of equipment to feed the engine. At some point the "not-a-pilot" lands and swaps the footballs.
Likewise one can carry a five gallon gas can-- secured with bungees or a strap-- in the baggage compartment and refill when convenient. There is no rule limiting how much fuel you carry, just how big the tank is. Same with a clock, mount it on the instrument panel and it counts as part of the empty weight. Wear it on your wrist it does not.
There is no requirement under 103 for any gauges, airspeed, altitude, compass. Personally, if weight were tight I'd just have an EGT / CHT, calibrate the throttle on the ground, and use smartphone apps for the rest. Mount a bracket for your device on the panel, the mount counts against the empty weight, but a cell phone or tablet in the mount does not.