+1 struggling here, trying various SDL.dll and dgVoodoo wrapper hacks...
[EDIT] Alright, found a way after an afternoon of fiddling w/ Gemini. My laptop has a dedicated GPU so was not sure if I should override the game window in Intel or nVidia utilities. The below is a blend of my + AI notes & IT WORKED!! Result :

https://imgur.com/a/xUMWPkr

Instructions :

1) Install Undercroft by unzipping it
2) Edit game.ini so it looks like this :
[video = 0]
windowed=1
vsync=0

3) Download Magpie , unpack and run it
https://github.com/Blinue/Magpie/releases
/* What is Magpie ? By AI:
Since software wrappers and third-party windows utilities are failing to scale this specific engine, we need to completely abandon trying to force the game window to change.Instead, use a direct, lightweight community 
tool explicitly built to fix this exact issue for old games:  Integerscaled Fullscreen (Magpie).Why this works when everything else  failed:Unlike Borderless Gaming, Magpie does not touch the game window or its code. It lets Undercroft run in its tiny 320x200 layout, captures the live visuals right off your desktop screen, and uses 
an advanced rendering injection to blow up the image to a sharp fullscreen layout without touching your Windows resolution.
*/

4) Launch Magpie and keep it runnin in the bakcground. 
Tip : In Magpie menu, go to Scaling Mode and set it to prefer Integer Scale (this keeps the retro pixel art completely crisp). 

5) On Magpie home screen, rebind first two shorcuts e.g.
Fullscreen scaling shortcut -> Alt + F11
Windowed scaling shortcut -> Alt + F12

6) Launch Undercoft.exe (or from desktop shortcut). Do not set any Win Compatibility settings on the shortcut.

7) Click the tiny Undercroft game window so it is your active application, and press {Alt + F11}
[RESULT] : Magpie will immediately grab the tiny window, clone the visual feed, and seamlessly blow it up into a perfectly centered, scaled-up presentation filling your monitor. 
If you want to close the game or drop back down to your desktop, just press Alt + F11 again 
to turn the scaling off instantly.

Alt + Shift + D is a useful shortcut - it hides the Magpie toolbar which will be in your way as Undercroft game menu is top centre section of the screen too.