These are some good candidates for your last slot of your team:
Snorlax - Is a static encounter and is caught fairly early in game. Covers the normal STAB you’re missing without Dodrio, hits hard, and tanks hits. The only noticeable downside to him is his abysmal speed stat, but he takes hits so well, it isn’t that bad
Persian - Very frail but insane offensive stats and speed. When paired with Slash, due to a problem in the algorithm of gen 1, he almost always crits.
Dragonite - Not the greatest movepool, but what it lacks in moves, it makes up for in his unique typing of Dragon and great stats. Unfortunately, you need to gamble all your money away to get him, but he’s also a great trophy Pokémon.
Tauros - Basically Persian but way better and once you teach him Hyper Beam you pretty much can laugh your way to Champion Blue because the elite 4 is so easy with this thing.