You're going to need the TTO conversion from the ingame bananas downloader
Also, the SimpleAI from the downloader, to imitate how the old AI worked.

Other than that, check this guide to see how to install the original graphics, which come separately
https://steamproxy.com/sharedfiles/filedetails/?id=2442756996

This is what you need to change in the settings to get closer to the original experience

Interface > Show the newGRF's name in the build vehicle window: ON

Accounting > Infrastructure maintenance: OFF

Vehicles > Physics > Train acceleration model: Original
Enable wagon speed limits: OFF

Vehicles > Routing > Disallow train reversing in stations: OFF
Automotive reversing at signals: OFF
Forbid trains from making 90 degree turns: OFF

Disasters / accidents > Disasters: ON
Recessions: ON

World generation > Landscape: Temperate
Land Generator: Original
Tree placer algorithm: Original
Start Year: 1930
Scoring end year: 2030

Environment > Authorities > Local authority attitude: Lenient. Listen, this is not how it was in TTO, but, being ♥♥♥♥ blocked by the authorities is not fun. This is the first of two non-vanilla settings im going to recommend because playing without them aint great.
Allow bribing: ON
Allow buying exclusive rights: ON
Allow funding local road reconstruction: ON
Allow funding buildings: ON
Allow town controlled noise: OFF

Environment > Towns > Town cargo generation: Quadratic

Environment > Industries > Economy type: Original

Environment > Allow more realistically sized catchment areas: OFF

Environment > Ingame placement of trees: Grow but only spread in rainforests, in case you want to play a tropical start, which, wasnt in the original TTO, but im not your boss. If you want tropical go ahead

This is not something that was in the original but i suggest you turn it on regardless because of how important this can be.
Interface > Viewports > Show path reservations for tracks: ON

This is a section dedicated to the things missing from OpenTTD that'd make it better to play the original TTO

1: Farms being a monolithic entity as opposed to a small piece surrounded by farmland.
2: With the TTO conversion, trains dont lose max reliability over time, nor gain them when they are first introduced. This makes the experience feel really stale.
3: Profiles for the settings so i can quickly switch between them
4: The removal of the kirby paul tank since it wasnt in the original transport tycoon
5: There was no noise mechanic in the original TT. I wish the TTO conversion would turn it off