You could reset your monitor to 640x480 while you play the game

It’d probably depend on the game, but see if there’s any source ports for the games in question. DSDA Doom, for example, adds modern wide-screen support to classic Doom and has heaps of fantastic options for different compatibility levels for different custom map sets. GZDoom is also an option if you want gameplay mods (shame it’s so inefficient lol).

You can also see if there’s any wide-screen patches for your games on the PC gaming wiki.

I have used this app when using emulators and it seems to work well https://store.steampowered.com/app/993090/Lossless_Scaling/

Gonna need more information here

Are you playing PC native games? Run them through dosbox to get access to its scaler and settings (you can run windows 95 in dosbox if needed)

Are you running emulated games? Jump into the video settings and adjust the scaling configuration

Are you running console games? Gonna want to get an OSSC or retrotink

Edit: just seen the post title says PC, dosbox is your solution then

If you’re using Windows, DXWnd has some options for improving the graphics in games. It may not work for every game but it’s probably worth checking out.

It’s a matter of time before we can use real-time AI upscalers. Nvidia (et al) have been working on this quite a bit.

In the mean time, the two options you mentioned are it.

Dosbox

You can always just turn your screen resolution down.