1972 CR400 Spark Arrestor

so a friend wants to ride with me in the woods, but there is no spark arrestor on his bike
it is a 1972 he bought new, it originally had a huge slip on that he hated, so he put one inside and secured it with a sheet metal screw, that popped out years ago, now none of these were spark arrestors
so the question, how do you make this legal, the outside of the tail pipe (stinger) is 2"

A couple of options with varying degrees of cost and maintenance:

First – SPARKY, fully legal with USFS stamp and inexpensive. Used on ebay for +/- $20

Second – SUPERTRAPP, full legal, available for $50 - $80. Will require periodic maintenance.

Third – FISCH, may or may not be useful for a 2-Stroke – Screen will need cleaning often.

Last – The bike in the pic shows a 72, but the pipe looks like the stinger has been modified. The 72 had a much smaller and shorter stinger – which would allow the use of an old school cylinder looking SKYWAY Spark Arrestor / Silencer combo to be fitted.

thanks that's the input I was looking for
not sure if modified, he bought it new and said not modified