From Winkler there are a four disc brake system (DM 3600) available.
They are also sold by Oettinger for the same price, but Oettinger
has a subsidiary in US. But this system requires 15" rims. Actually
I don't know, if this system has ventilated front rotors, but
possible.

Winkler also sells front slotted rotors as a replacement for the
regular 14" system. I do not have the price on hand.

In South Africa VW sells their Microbusses with a 15" front disk
system. Should be available as spare.

Helmut
'80 Camper