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?
Modifying an existing chip is pretty much impossible. Chips contain structures on multiple layers, but you can only really work on the topmost layer directly. There also usually aren’t really unused gaps, components are placed as tightly packed as possible because if you can make the chip smaller then you can get more chips out of each wafer (because manufacturing costs depends mostly on the amount of wafers and amount of layers on each wafer, not on the amount of chips per wafer).
I guess in theory you could grind the old circuitry off the chip to then build up new circuits on the old chip, but that’d be much more challenging than starting from a new wafer and also not really offer up any benefits.