In the case of 13 ohm gauges common to standard panels, yeah they can look good as new on the inside and work just fine. This is where something like the RTE, other, aftermarket solid state regulator has much success. Just change it out like a light bulb and happy motoring again.
3 X 16 ohm gauges in a rallye panel is not the same animal. To open this fuel gauge will reveal black crunchy carbon build up on the ni-chrome winding and that bi-metal beam bowed at room temperature. It aint gonna respond correctly. That's why they say open it.
Then none of the fuel gauges will be correct with aftermarket senders anyway.
I apologize to the owner/thread starter for any chit storm I may have created. Good luck with it.