If so, roughly what year electronic could be user by hobbyists?
Would it be better to abandon the existing transistors and etch the gaps or could the old be integrated?
Would it be a green process or would the process be too wasteful for any potential gains from re-using old electronics?
Hypothetically, it might be possible, but there aren’t practical gains to be had from it. Easier to fabricate a new chip.
The optimal design is unlikely to be a straightforward modification of an older chip.
Even if it were, modification would add more variables, where we’re constrained by yields today.
And our processes today aren’t designed for it.
EDIT: That being said, you might be able to repurpose an older chip in a piece of electronics that uses multiple chips in a situation where you’re unable to get the older chip for some reason. I can’t think of many scenarios where that’d be economical, but hobbyists have certainly reused components from older electronic devices.