Okay, so if we assume that
>You're playing Gold or Silver, not Crystal, and no real life trading is involved, even with the Time Capsule
>No glitches
>Only Gen II Pokémon pre-League count
You can obtain 71/100 new Pokémon.
Pichu, Cleffa, Murkrow, Misdreavus, Sneasel, Slugma, Magcargo, Houndour, Houndoom, Porygon2, Elekid, Blissey, Larvitar, Pupitar, and Tyranitar are exclusive to after the League.
Two starter lines remain inaccessible
Spinarak, Ariados, Phanpy, Donphan, Gligar, and Mantine are exclusive to Gold while Ledyba, Ledian, Teddiursa, Ursaring, Delibird, and Skarmory are exclusive to Silver
Ho-Oh is exclusive to Gold pre-League and Lugia is exclusive to Silver pre-League
Celebi is completely unavailable.
Now if trading is allowed between all Gen I and II games, but we're still not using glitches, the number of Gen II Pokémon available pre-League potentially becomes 90/100. And if we count Mt. Silver as a Johto location, we can still say Misdreavus, Larvitar, Pupitar, and Tyranitar are Johto Pokémon. The only Gen II Pokémon truly "exclusive to Kanto" are Murkrow, Slugma, Magcargo, Houndour, and Houndoom. Even though an Up-Grade can only be obtained in Kanto, Porygon2 is a trade evolution and therefore has no native region.
But if we're counting Gen I Pokémon, players can obtain more than 210 Pokémon pre-League in Gen II with trading compared to 149 in Gen I with trading.