How hard is it convert a 71 standard beetle from a ball joint front end to a king pin front end for a baja?

The frame head and beams are different so can't swap one to the other.
The easiest way is to cut the link pin tubes from the shock towers and brackets and then make custom shock towers and weld BJ mounting brackets back on.
If the tube diameter of both beams is the same and the torsion leaves the same length you could also weld link pin sway away adjusters into a BJ beam and fit link pin torsion leaves and arms.
Finally, you need to weld a link pin frame head to a BJ pan or just whack a link pin pan under your BJ bug.

Least that gives you a few options

I figured it was going to be a frame head swap. But I can just put a LP pan under the 71??

yeah body swap is the easiest method

Rear body mounts need to be relocated...