hi,
does a 16v engine fit straight into a 8v car? (mk2a 2000)

As far as I'm aware no, you'd need all the loom from the 16v also, aswell as the ecu. You'd then need the ignition barrell and locks from the 16v so the key matches the ecu.
You can leave the locks but have 2 different keys for engine and doors.

dam i was thinking bout this if and when my engine blew up lol..

Is that all you would nee to replace as well as a exhaust system from a 16v also?

If so its still doable and i might give it ago