... depends upon if its an emergency ie you need to get home or not.
There is no 'good' knot except if you splice both ends to join. I think I read some where, if you have a knot, the line strength is halved so you really need to nurse the kite back to the beach once you have re-joined in your emergency.
I experimented years ago, and one of the joins I did was make two loops with a bow-line knots then join togeather. Another one was bowline loop one side, then a bunch of granny knots the other to make sort of lump for the loop to hitch around. Then I did a few fishing knots like 'locked half blood knot' and even normal old hitches.
The worst one was the granny knots and bowline loop, the granny knots bunched togeather to up to make a lump unravelled and snapped while under tension (kite looped and crashed as it was a front line).
Anyway, my conclusion is the only proper way is replace the line, then splicing with sheaths as protection and at a pinch to get yourself home by nursing the kite, a couple of bow-lines looped togeather.
cheers Robbie