I have to agree with CryoTheMayo to a degree. Ash's Pikachu has many feats that put him on an at least pseudo-legendary level, as shown that he was still able to keep up with a Mega Gyarados BEFORE finishing it off with his Z-Move. So it's safe to say that Ash's Pikachu has better feats, considering this is like Gen 2 Pikachu
Assuming Red's Pikachu can know more than 4 moves, the only thing I can think of that will give Pikachu trouble is the TM moves that Red's Pikachu can learn, most notably his stat amp moves, but Pikachu's Agility can help him dodge literally everything Red's Pikachu has. In Gen 2 Red's Pikachu has Curse, Defence Curl and Double Team, which can very easily be dealt with by Ash's Pikachu. Curse will make Red's Pikachu slower, which isn't good, defence Curl is rendered useless with Curse and can be bypassed with any of Ash's Pikachu's Special Attacks + Thunder Armor, and Double Team is rendered useless with Curse, since Double Team relies on the user's ability to "create illusionary copies", which appears to be made through rapid side-to-side movement. This won't work since Ash's Pikachu has Agility, which will make him many times faster than Red's Pikachu, butalso Curse slows down Red's Pikachu even more.
Plus Ash's pikachu has 2 methods of immobilization: Counter Shield and Electroweb which makes it even MORE unfortunate for Red's Pikachu
Also, if I remember correctly, didn't Ash's Pikachu beat a Raichu by absorbing it's electricity, even though the Raichu was vastly superior to him? There's nothing stopping Ash's Pikachu from doing the same thing, which Red's Pikachu cannot do in-game.
All in all, I can't see Red's Pikachu winning at all