I wouldn't call a 6-speed Helicomatic "rare"; actually, all the Helicos I've come across except 1 (a 5-speed) have been 6-speed. The Treks all had Sedisport chains as mentioned. I replaced the worn Sedisport on my "frequent rider" (Ruby) with a KMC Z-chain. I forget the model designation, but I see "R6" on the outer plates.
This one[/url is different (mine does not have those windows), but I reckon it would suffice for any six-speed friction ops.