I cut my screen-type spark arrestor out of my DS650. I also modified the end cap pipes a little. Someone said re-jetting or some carb. mods may be needed with the change of airflow and stuff. Is my valve going to crack? What should I do now?

I don't know if that is enough of a mod to worry about the jetting. But if it's a concern to you watch your plug for color if it starts to turn white then rejet fater. The best color is a dark tan for duning.

You shouldn't have to rejet as long as you have not modded the airbox at all. But to be on the safe side, do a plug check and see hows its running!