I have just received a package from VWHeritage containing genuine parts that may be of interest to anyone about to fit a radiator, who wants to source alternative parts.
The foam rubber gasket for the top and bottom of the radiator (part number 431819225) is an adhesive-backed strip of medium density black foam rubber 1360mm long (one strip cut in half does top and bottom), It is 15mm wide and 15mm thick. Having seen the genuine article I can confidently say that any foam weather-strip from the hardware store would suffice. I have used similar materials for weatherproofing a tin shed.
The rubber mounting washers for the radiator (part number 171121276D) are the same as those used to attach beetle running boards. They measure 24 mm OD, 7 mm ID, 4 mm thick.
Cheers, Roger.