Some questions about the different years

[MuRCieLaGo]

Hi,

I got some questions.

1. At what year was introduced the convertible ZR-1?

2. I saw the engine had a boost in 1993 of like 20hp. Is it internal or external upgrade? I mean, could I buy a 1992 and easily get these mods?

3. What are these exterior restylings it had over the years?

Thank you!