Whether we need permission or not depends on the licence.
If it's an open source licence, they can't legally stop us.
However it's still best to ask, even if you don't technically need to.
It's better to have the developer's blessing and for everything to go amicably,
than to potentially upset the developer.
A really good developer would say "go ahead, port all my games,
I'll even give you some help if you get stuck".
Though it's understandable if they don't -
people have this odd tendency to get very attached to projects they worked hard on
Although if the developer did get upset, they probably picked the wrong licence.
If you go open source, you usually have to expect people to just make derivatives without asking -
as long as they follow the rules of the licence they haven't technically done anything wrong.
That's why it's important to think carefully when picking a licence and make sure you use one that genuinely reflects how you feel about your code.