What system? GBA, DS, PSP and PS2 all appear to feature such games.
Also what do you want to do with them? Normally we get people coming along and wanting to change the cards in their deck. This is actually quite hard to do if you want the AI to still be able to do anything good (I usually link this article https://magic.wizards.com/en/articles/archive/duels-planeswalkers-all-about-ai-2014-02-13
), however if you want to play against other players that is a different matter.
If you just want to change some graphics, text and other such things then that is fairly normal ROM hacking we can teach in fairly short order.