My personal guidelines that I usually follow are as long as:
1
You own the version of the game that you are emulating.
and
2
You own the hardware that you are emulating.
then there is no problem.
Exceptions I make are when the games in question were never released in my region to begin with or if that version of the game is no longer available to buy at a reasonable price.
On point 1
I know there are various re-releases of certain games (Sonic 1, for example, is available on various digital download services) but I still believe that as long as the original version is owned then the original version you can emulate. No upgrading to the perceived fancier version with extra bits just because it's "basically the same game". Those extras are for those who own that version.
On 2
If your hardware (Inc game cartridges, discs) is broken: tough titties. Repair that shit or pony up for a replacement.
Again, just my personal rules.
1
and
2
then there is no problem.
Exceptions I make are when the games in question were never released in my region to begin with or if that version of the game is no longer available to buy at a reasonable price.
On point 1
On 2
Again, just my personal rules.