Is there anyway for a DM to assign Boons, Gifts and Background replacements in D&D Beyond? I can't seem to find anything anywhere.

Not currently. I know, a while ago, they talked about adding boons to the character sheets but I haven't seen or heard anything about that progress since they. I know it's been a very requested feature.

I would think most of them can be simulated by creating custom feats and applying them to the character(s) in question.

Some of them yes, but if you wanted something like adding a x level spell slot no. 

Looking over the boon list, a lot of the spell ones could be hard to do with a feat if not impossible. 

👉How to quickly and easily implement almost every single Epic Boon on DDB.👈