Came across a very good condition carbon Ksyrium SL front hub. Do I have to lace it with the $5 mavic spokes, mavic nipples and the Ksyrium SL rim? Or can I just use some DT Aerolites or CX rays, generic nipples, and a Kinlin rim?

Hub is useless without it's proprietary mavic spokes and rim. You cannot lace to a regular rim

It's not entirely useless! You can pull out the (usually SKF) bearings and use them for something else. Once the bearings are out, you can cap the bottom and top, perforate the top, and use it as a salt/pepper shaker, ala Chris King!

Main issue is that the hub is set up for straight-pull spokes(no j-bend). I haven't been able to figure out a way to get a j-bent spoke to engage the hub in a way that I'd be ready to trust long term.
I have an earlier version of Ksyriums, and they'd happily take generic straight-pull spokes, but I can't tell for sure if they'd fit yours. You'd might have to fit spoke washers to widen the head or some other kludge.
Obviously, if you manage to reliably anchor regular (straight pull) spokes, you're then free to use whatever rim and spoke nipples you can come up with. Rims don't care much what hub they're laced to, as long as you get the required spoke tension.

You can have lbs send the hub in to Mavic and they will rebuild it for about $300. New wheel is about $600 if I remember correct.

Some campagnolo and mavic wheels use everything proprietary, from spokes to rims. So no way to use parts besides rebuilding other wheels from the same brand or as somebody said, use them as a salt container or paper holder o something.

They are proprietary 'straight-pull like' spokes. They don't fit. Like I said in post #2.

Assuming he has the rim, which he doesn't, nor the spokes.

I expect the $300 covers Mavic's providing the new rim and spokes and relacing them to his original hub.