I was thinking of optocouplers, as them will give us total isolation of the Enterprise side, then it can see mere switches connected to its Joy1 port.
The program needs little modifications to light the leds, it can run almost the same.
I remember a Midi adapter with optocouplers I made for my Amiga 500+, and Midi operates at 31250 baud, fast enough for this project.
For the STROBE signal(RTS) it can be connected directly as it can be referenced to +5v or 0v the same with the 10k and 82k resistors like in the Boxsoft's.
Then I need 6 optocouplers(4 ways and two buttons), but I need advice to select the best type for this task.