You should determine the number of strands and gauge of the existing conductor before doing anything.
Structures under 75' are defined as Class I structures so the ground loop could be as small as a #2 or 29 strands, 17 gauge copper.
Structures over 75' are defined as Class II structures requiring between a 1/0 and 2/0 or 28 strands, 14 gauge copper.
BUT these are just the minimum required by the standards and the architect/engineer/owner may have required something completly different like a 4/0.
If there is enough slack you could use a mechanical splicer like a parallel or butt splicer or even an exothermic. Count the strands carefully and determine the gauge. Note: Conductor used for lightning protection should be labeled for lightning protection use by UL.