27.0 crown race on a 26.4 fork? Probably best to get a new headset which fits rather than trying to track down a matching crown race. No readily available adhesive will hold against the forces involved, so don't waste your time trying that route. And yes, the play is probably the fork crown moving back and forth inside the crown race, something which will eventually destroy your fork even if you could ride around the horrible handling.
OK, thanks for that.