By repairing it instead of modding it..? Am I missing something here?

Open console, troubleshoot components, replace components as necessary. You might be overthinking this one or your question might not be specific enough. Modding is the act of replacing factory components with ones that are non-standard, repairs are not necessarily mods.

Mod means modify. If by mod you mean change some of the components for new components, and you don't want to do that, then you're not going to be able to fix any hardware issue.

If you replace the defective parts with OEM replacement if they are available, then you haven't modded it but repaired it. Even if you use 3rd party aftermarket parts, might be modding but functionally it's unmodded. It's only if there are extra lights, buttons, major visual component or a modification in software behaviour is when I'd consider the console functionally modded.

Old Sega consoles like that probably need their electrolytic capacitors replaced. I’d say like 80% of non-working Sega consoles I’ve had just needed the caps replaced.

I guess it would depend on what's wrong with it? You shouldn't need to "mod" anything just to repair it, although you may possibly have to replace some parts

Give some details about what's wrong could be a quick cleaning issue or you're breaking out the soldering iron.

Are you referring to how to repair a console with original parts?

Maybe the logistics of it like how to source parts in a way that's not going to be outrageously expensive?

Uhh... Depends on what is wrong with it. Is nothing happening when you try to turn it on, is it having trouble reading games, or does a controller port not work?

Some things can be pretty tricky, like if it needs a new laser in the disc reader. Whatever the case is, you should definitely start by popping the case open and to see if any capacitors are bulging/leaking/burst