IIRC, Arc/HN runs on mzscheme because they rely on mutable cons cells that newer versions (racket) don't provide. Could they run under new rackets by just telling racket to use a proper scheme?
As a Racket author, I can tell you that we provided them with a hack to enable mutation on arbitrary conses, so they can upgrade whenever they wish, and I don't know whether they have or not.
There is an mzscheme language mode, but it specifically does not include set-car! and set-cdr!.