120 BCD? Whatever that is, it isn`t very common, will probably be tough to find your guard. Since you say "replacement" chaingurad, I take it you used to have one and it got trashed somehow? Maybe you can fabricate one from a piece of sheet metal. The idea mentioned above about cutting the teeth off a bigger chainring would be the simplest way, but sourcing a big enough chainring for that bolt circle might prove as hard as sourcing a chainguard.