Like I wrote in this post, Corsican cattle (or Corsicana) and Steppe cattle probably are the most important founding breeds of Heck cattle. The two cross animals considered to be the first Heck cattle bred by Heinz Heck both were 75% Corsican cattle. And indeed if you look at members of this breed today, there's still a large and undeniable similarity with many Heck cattle. Bear in mind that the modern Corsicana have been crossed with dairy breeds from the European main land to a large degree during the last century. So the Heck brothers worked Corsican cattle that were more primitive than those we see today, without deviant colours and so on.
Some examples of Corsican cattle (sources are visible in the URL of the photos):
Corsican cattle is a rather small breed, and also the horns are much too small to fit the aurochs. The positive phenotypic feature this breed contributed is the very aurochs-like wildtype colour. But it also has grayish or beige variants in cows and bulls with prominent saddles, and we see exactly the same colours in Heck cattle. Also the horn shape greatly resembles that of many Hecks, just smaller. Some Heck cows, like an individual I saw at the zoo Haag in Austria last year, are almost indistinguishable from Corsican cows.
In my opinion, one could easily breed "Heck cattle 2.0" by simply crossbreeding Corsican cattle and Hungarian steppe cattle. Would be pointless (= not very aurochs-like), but fun.