Suggestion for how to put the old version up

[Acer Spacer]

A good idea of how to leave it up would be (my suggestion) something similar to how "A valley without wind" did it or as a dlc.

Background info; first game came out then later second game, it was sequal that relied heavily on the old game so it was super cheap for owners of the first game and then onward both were sold in a pack and you couldn't get them individually.
Halcyon 6 also did something similar

Another way would be having it as a free dlc that is a self contained folder containing the old version