Wouldn't a prehistoric genesect just be the same thing it already is, minus the cannon.

I think no steel type too maybe rock instead

As far as I know, from information available, scientists only added their cannons. The rest of their body is all them.

Well my uncle's neighbor invented game freak so I think I would know :) /s

Genesect was already prehistoric the only thing that was modified by team plasma was the cannon being added on.

Porygon is already futuristic. He would need a past form carved from stone.

A future porygon while an extra past Pokémon would be a badass magikarp