If they match in form I see no reason why they cannot be substituted.
Essentially they are put into the design to get the chain wrapped around the driven, and loaded, components. The crank and cams being these parts, the idlers try to prevent the chain contacting the nose of the sprockets even when run with slack in it. If a chain meets with the top of the sprocket teeth it'll almost instantly snap it, or certainly overload it severely.
In addition, by making shorter chain run as it's divided into several sections, this serves to change the frequency at which each of those runs will resonate, by shortening the sections the frequency should move to a point that occurs above the operating inputs from the engine, so avoiding the chain itself getting to a point at which it would be detrimental to its own structure and fatigue / lifing.
The idlers don't have to be anything fancy as long as they have the ability to take the same bearing load in use.
