May sound crazy & I have not done this with 50 year old carpet... I have done this on 2 vehicles in the last 6-7 years, most recent was last week.
Pull the carpet, load it up in the truck, head to the local car wash, soak (really soak it) the carpet with "tire cleaner" (Simple green if the car wash does not have it on the dial), let it set for several min, scrub any stained areas with a brush, turn to soap & hose down really really well, flip to rinse & spay until water runs clear. After all this take it home & let it dry, may take a few days.
The 98 GT convertible carpet would have looked like new if it had not been sun faded. Even removed all of the coffee stains from the saddle carpet!