If it's walking, it's old enough to survive.

The crying is normal, because it's not normal for them to be alone. Cats are social animals, they're just more independent than dogs. At that age, they're not away from their mom for long, even then there's usually siblings.

Things you should know before taking thing on. They are born as needy as a human newborn. A lot of the advise is the the same, like you can't spoil a newborn. They have their emotional needs too. Having a good bond with the mom (even seragate) feeling loved and secure is the key to them having healthy bonds as an adult.

They can not maintain their body temperature. A newborn can die of hypothermia on a warm spring day, if it's not warm enough. They need to be kept in a warm room, provided with lots of cuddles, and blankets. Sasha felt better when she had a plush doll to cuddle with when I couldn't.

They need to be fed every 4-6 hours, if memory serves. I would start weening immediately because once those teeth grow in they'll chew off those bottle nipples almost immediately. I was unable to find a place that sold the nipples separately, and buying bottles every feeding or 2 adds up.

They are also not born with the ability to use the bathroom on their own. You have to take a damp washcloth and wipe their bottoms to trigger a reflex.

Cats are clean because their mamas gave them high standards by bathing them constantly. You'd have to reproduce this, before and after meals, naps, bathroom, being touched. You'll be bathing them like 100x a day.

And finally, there's an ongoing debate on whether cats are actually domesticated. Basically cats long ago learned to behave in a way that gets them a free ride. That knowledge has been passed down from generation to generation. If you have another adult cat, they will teach it, if not it's up to you. Do to the language barrier, it takes significantly more work to teach them everything from litter training to playing nice. Teaching them not to bite is one of the hardest.

If you don't feel up to the task, there are fosters who specialize in this.