If the speedometer meter is working then it can't be the sender. The sender feeds both parts (meter and odometer). You should have a 'newer' style speedometer (verified if you have the sender as shown in second photo from Mike) , the chrome ring is fairly easy to get off these. Just find the narrow portion and wiggle it out in one of the openings at the 3 or 9 pm positions. Slowly and carefully turn the ring until it is off. Note carefully what parts go where. There will be a chrome ring, black plastinc ring, clear plastic ring and a paper gasket.
The newer syle odometers are driven by a DC motor, old style is stepper driven. Both use gears.
I would try again what James suggested, play with the reset knob to see if the odometer will wake up.
Gerrit
Gerrit's Ferrari Dino 308GT4