The issue here is the unique shape of this pre-formed line. Exiting the tank, the line makes a tight 180° turn followed by a 90° to the fuel filter. Installing generic fuel line will likely result in kinking at one or both bends unless some strong supporting spring is employed or the use of a few 90° brass fuel line fittings. I adapted the latter but it was a tight fit what with the additional hardware fighting for limited space beneath the tank.
I eventually found the OEM line in the Netherlands at cmsnl.com, but last week I stumbled upon another supplier in this eBay listing from seller getor-shop in Germany. There's several available at about $39 each including shipping to the US. Having a spare on the shelf might be a good idea for all us Gen 3 owners.