Assuming you have a minimum of the 4 corner pieces of sisley 4x4 roof rack, one can fabricate one from scratch, or replace rusty bars and brackets. The pictures below show the technical specifications of the bars and the materials you need to manufacture them. You can find some of these parts on ebay. Most are not available through the dealership anymore. These can all be fabricated also aside from the 4 plastic corner pieces and the plastic end bar covers, though they are not strictly necessary.

Part Numbers:
Roof Rack - 7606591
Rear Bracket x 2 - 7613122
LH Front Bracket - 7613118
RH Front Bracket - 7606634
Ski Holders x 8 - 7606590

Fabrication Materials and Measurements
Screws no 4 Allen M6 - x 6 @ 25mm long
x 10 @ 20mm long
Rear Brackets - 3mm Mild steel plate 270mm wide
Front Brackets - 0.9mm mild steel/stainless steel alternative plate. Insulation tape on rear to aid adhesion and avoid scratching paint
Horizontal Bars x 6 Mild steel- 975mm x 10mm x 20mm with 2mm indents each end, 1 bar has 2 holes, 90mm from each end for the rear brackets.
Side Bars Hollow Mild Steel 790mm x 10mm x 20mm Holes at 15mm, 265mm, 525mm and 780mm