Unfortunately, the Yamaha Reface DX isn't a DX7 replacement. The DX7 uses six-operator FM synthesis; the Reface uses 4-operator FM. I believe that the Reface sounds -- while neat -- aren't as complex as the DX7 sounds.
Perhaps the Reface DX matches up to a DX21, or something like that. My memory isn't reliable, that far back.<g>
There _is_ a DX7 software synthesizer -- Dexed -- which has received good reviews. Since it's free, and accepts DX7 patch files, it's worth checking out.
. Charles