This is not strictly sculpting but close. I have a 120 mm Robert the Bruce figure on horse and the lower leg of the horse’s foot is broken and gone. The good thing is the majority of the leg is hidden under cloth (forgot the right word) I was thinking of molding one of the other lower legs and using that as replacement. So my question is what is the easiest may to make a copy of a resin piece. I do have latex rubber so that maybe a start any help appreciated.

If you can position the good foot in some way so you can hold it in a little box to add the latex in place till it cures you can make a mold of the foot.
Then you can use resin or plaster to pour in the mold.
You can also look at using something like milliput to carve your own without a mold. A horse foot isn't super complex. If you go this route I'd make the foot a bit big or rough and sand it down with files and sandpaper.

Attaching the leg part to the main body should be done with a pin and hole method. Drill a small hole in each side and fix a short piece of wire to hold the pieces together.