Someone on here makes and sells replacement parts. Try whatever glue you have, and if it doesn't work buy a new one. I'll try and remember to look around, but I'm sure someone will chip in and find it before I do.
Edit: quick search found this thread with some good suggestions.
https://www.canadiangunnutz.com/for...y-840-Forend-spacer-need-one?highlight=Spacer
using plastic was a smart way to reduce weight
No, it was a smart way to keep people buying a part that gets heavy use and is prone to breaking.
Western Gun Parts has a metal replacement that will last the life of the gun.
Nothing will really repair that other than a replacement part...