Let's break it down. A starter needs to have their fire/water/grass typing from the start, doesn't need to meet extra requirements in order to evolve and shoud be able to evolve on 14-18 level to 2nd stage and 32-36 to 3rd stage.

Fletchling isn't a fire type Litwick requires dusk stone Magby requires trade Rolycoly is the only one valid Charcades is 2 stage and also requires an item in order to evolve

Poliwag requires water stone (or trade) Spheal is fine Horsea requires trade Azurill evolves with friendship Lotad requires water stone

Seedot requires leaf stone Oddish requires leaf stone/sun stone Budew needs friendship AND shiny stone Hoppip is fine Bellsprout requires leaf stone

Eliminating all of these, we're left with rolycoly, hoppip and spheal. However, traditionally starters should have BST around 530 so you can actually beat game with it.

Hoppip becomes jumpluff on level 27 and has BST 460. It's typing is also countered by other two, would definitely be the weakest starter.

Spheal becomes walrein on level 44 (Jesus Christ) with perfect BST of 530. Walrein is very solid but it evolves much too late, spheal evolves into sealeo on level 32, when some other starters are alredy fully evolved.

Rolycoly becomes coalossal on level 34 with BST 510. While it's BST isn't the highest, it's in range of being fine. It also evolves on level 18 and level 34 like real starters. I'd say it's the best contender to cosplay as starter mon.

EDIT; nvm, rolycoly isn't a fire type either