I love the idea, but any mod before 1981 will be problematic because that year the Concorde Agreement gave FOCA the right to negotiate TV contracts.
This the 1984 Mod to be sold legally and go mainstream would require permission. This would be quite a process which I am unfamiliar with.
If you make the 1984 F1 Season off the radar and create some sort of donation funding from backers for your time and it stays within a select group of people, no one is likely to hassle you.
However if it goes mainstream and its the mod everyone is driving, and you have charged for it... I imagine there will be problems.
Timo Verneersch will know more on the legal side.
Would love to drive the 1984 F1 mod.