I would actually place it in Group F-1. If they are doing engine repair, then Group S-1 would be applicable. Group F includes assembling, disassembling, finishing, and repair not classified as Group S or H. If the work is limited to just body work, then I think the Group F-1 is more appropriate. Regardless, the allowable area and height in stories between Group F-1 and S-1 are very close, with the F-1 being more restrictive in some construction types and Group S-1 being more restrictive in the others.
IMHO, either can be used for a body shop; thus, use the one that is most advantageous.