I NEVER imagined I'd have a reason to come back to this card game, but this simulator having the way more fun to play with older cards, and more importantly the hot seat functionality allows me to make custom rules for more interesting matches. So thanks for the inspiration. Just here to report some bugs; based on the comments I guess development for this has stopped, but I'll share what I found anyway.
1. "Old Rod" lets you get a Trainer card if you flip 1 heads and 1 tails.
2. "Pokedex" and Hypno's "Prophecy" attack don't return the cards you rearrange to the top of the deck.
3. "Devolution Spray" returns the Evolution card to your hand instead of discarding it. Pokemon who devolve by any effect also have all of their damage counters removed.
4. A "Buzzap" Electrode being used as Energy can't be discarded for retreat costs, and can only be treated as 2 Electric Energy.
5. Alakazam's "Confuse Ray" attack ignores Resistances.
6. The damage for Tauros' "Rampage" attack doesn't scale correctly. It's able to do hundreds of points of damage with this attack.
7. Dark Magneton's "Magnetic Lines" attack causes the Energy being transferred to the opponent's Benched Pokemon to take its place, acting like a Pokemon that has no attacks and can't retreat.
8. The 2nd Energy "Super Energy Removal" discards is chosen from the Energy in your opponent's discard pile.
9. When discarding Energy to use a "Max Revive", the 2nd Energy card you choose doesn't get discarded.
10. Ditto's "Transform" Poke Power doesn't copy the Weakness and Resistance (or lack thereof) of the Defending Pokemon.
11. If a Pokemon that evolved from a "Mysterious Fossil" gets KOed, that "Mysterious Fossil" is still treated as a Basic Pokemon while it's in the discard pile.
12. Evolving a Meowth into Persian (not Dark Persian) KOs it.
13. Diglett's "Dig Under" attack only targets Benched Pokemon. You can still select the attack even when the opponent has no Benched Pokemon, at which point the game can't continue since you can't choose a Benched Pokemon to attack.
14. After Dark Arbok uses its "Stare" attack on a Benched Pokemon with a Poke Power, clicking on that Benched Pokemon prevents the game from continuing, as it brings up a menu that can't be closed with the only option being "Attach", which does nothing.
15. Charizard's "Fire Spin" attack doesn't let you choose which Energy to discard.
16. Damage from Raticate's "Super Fang" attack rounds down instead of up.
17. Dark Muk's "Sticky Goo" Poke Power and Dodrio's "Retreat Aid" Poke Power don't work.
18. "Transparent Walls" doesn't work.
19. The Promo Arcanine is being treated as a Water-type Pokemon.
20. Not a bug, but the old rules apparently made it so Confused Pokemon had to flip heads in order to retreat successfully; that feature's missing here.