The Flexible parts of the Jacobsen/Ford driveshaft are nothing more than Reinforced Neoprene Sheet material. It can be purchased, cut and drilled to replace any of the flexible bits. If Necessary it can be doubled up for more strength.
If you find the right supplier you can even select the reinforcing material.
I used to get it as scrap bits from a local oilfield gasket shop. The material is also similar to swather canvas, and similar materials.\
If a complete replacement is required due to hard parts damage, Lovejoy couplings could be used, and while not original, would work fine.