A treasured glass ornament is not easily replaceable. This is a guide about repairing glass ornaments.
Does anyone know how to repair an old glass ball ornament? It has sentimental value to me.
By p from RI
Glass ornaments are so delicate, expandable foam could break it more. I'd leave it alone and hang it with the hole facing the tree so it can't be seen. Or, put some clear tape on it and color the tape. Or, find an ornament with similar color, break it and use a piece to cover the hole and glue on.
Also, fyi, craft stores sell clear, plastic round ornaments that open so you can put something in them and then close it up and hang it. I've done that with sentimental ornament just to protect them. Good Luck
First glue the crack if it is still extant and in contact with it's mirror edge, that will help to strengthen it. Fill the ornament carefully with light cotton (pull cotton ball apart to loosen and lighten it). Put enough in the ornament to support the piece which is broken off, no more, do not stuff the ornament tightly. Use instant glue or costume jeweler's fast setting glue, place a very thin bead of glue on the edge of the broken piece.
Place it carefully in line with the broken edge of the ornament. Gently hold it in place until it sets. Leave the cotton in the ornament. Stronger ornaments you can take the cotton out, but you risk breaking the ornament if it is very thin. If it is strong enough and the paint is on the inside, you can take a sharp single edged razor blade and slice off any residual glue. If you even think it might be fragile, though, do not do this. Just dab any residual glue quickly with a clean Kleenex or Q-tip.
Perhaps Krazy glue a bit of decorative ribbon over the crack/hole ?
Don't use expandable foam! Even the slightest amount too much will shatter your ornament. The plastic ornament cover sounds like the best idea. Ask at a Hallmark card shop. I have seen them there.