Agree. You may want to take the stock off and put it in a black plastic trash bag and set it in a vehicle.
Field strip the gas tube assembly, and leave the top hand guard on the gas tube.
Put that assembly into another plastic bag for the same treatment.
The heat will cause the Cosmoline to melt out of the stock and handguard, allowing you to wipe it off easily with no chemicals.
Do this over the course of several days, occasionally removing, wiping off, and repeating.
This will improve the appearance a lot. Dents in the stock could then be steamed, using a wet paper towel and an iron, to lift them. Sand (if at all) minimally. Don’t go ham.
Wipe off one or two more times with mineral spirits. Once the stock is clean, you will have a better idea of the crack.
The cracked stock wrist could then be pried open and epoxied, and clamped together while drying for a seamless repair; and/or pinned with a brass pin.
Coat with boiled linseed oil, and repeat till you have the desired finish.