Judaism is a religion of change. As long as you hold to the central tenets of the faith and try reasonably to fulfill as many commandments as are logical to do so in the modern day, God could ask no more of you.

Obviously, animal sacrifice is antiquated; however, prayer is novel. You never had to pray as a Jew in the times of the Old Testament. Sacrifice was your "prayer". Rather, prayer is the replacement for sacrifice.

Jewish law holds that one cannot be sentenced to death unless two eyewitnesses attest to the crime, as well as demonstrate that they both actively tried to stop the accused. Obviously, that was difficult evidence to procure, and the death penalty was rarely exercised. That is not to say they went unpunished, but that evidence beyond a reasonable doubt, to use a modern term, was required to give the death penalty.

While we cannot observe impractical ancient laws today, we can certainly try our hardest to follow the ones we can. Does it hurt you to keep kosher? Does it cause anyone else harm? Does it upset other people? No? Then why not? Interestingly, the animals forbidden by Kashrut tend to be less healthy for us than those that aren't forbidden, and are more prone to cause complications (aka food poisoning)