KX65 silencer/spark arrestor

My son will be riding his KX65 at some point in the State Forests that require a forest service approved spark arrestor. What are the choioces? I'd like to just get one silencer that will work in the woods and still not be a drag on the bike when he takes it to the track to rip around on. I'm also fairly certain that at some point tihs bike will be receiving a new pipe to go with the new muufler if this makes a difference in recommendations.

Thanks for the help -

Mark