Was it glued originally, or did the maker rely on the friction fit between the liner and cover? I don't know, I've never disassembled a scabbard of that vintage.
If you must glue it (if it was originally glued) use hide glue. (Actually, for a job like this you probably want fish glue, with vinegar added to the water the glue granules are dissolved in.) Between the fish glue and the added vinegar, you get a bond that is more resistant to heat and humidity (and occasional rain etc). If you can't find fish glue granules, go to the grocery store and get Knox brand pure gelatin. Pain in the tucchus to work with the first few times, but you can cheat to make it a little easier.
Clean the surfaces to be glued. Have your glue heated to the namufacturer's recommended temp (use a double boiler setup if you don't have a glue pot). "Size" the wood and canvas surfaces (apply a very thin coat of thinned glue and let dry). Then assemble the scabbard and apply heat (I usually use an iron, with a folded hand towel in between) to the area to be bonded. The heat will soften the glue; when it cools it will be glued together.
Should you ever need to disassemble the scabbard for repairs or restoration, the process can be reversed. Contact cement etc are permanent (yay!) but often have long-term negative effects.