Hello everybody. 1970 commando. If I use the JS reed valve at the stock cam breather port

Can I get away with just leaving the non rotating portion in place and with no other mods like drilling holes in cases. going to be a normal street bike.

It simply won't work as well as it could. That's a fairly long and narrow path from the cam, so the lag will mean the pressure pulse will have passed before the reed will finish "exhausting".
You want the shortest path from the undersides of the pistons (the cause of crank pressure) to the reed as possible.

I removed the rotary disc and fitred a Mike's XS breather near the oil tank on my '70 and it works just great for a street motor.
No dispute that they work best mounted on the engine itself, and that's how I do it on tuned motors.
I've fitted reed valve breathers adjacent to the oil tank on several stock builds, and they still perform well.

Ok I will just do it without the rotating disc thanks. thats a small path I know with the 4 small holes in the cam no wonder it didnt work well.