Steel Assault is working OK for me (i3 4570T, ATI HD5450). However I had to used windowed mode, as fullscreen mode has some pretty nasty stuttering, maybe that's the problem you were seeing.
With windowed mode set to 320x240 and a 320x240 screen mode, the window title and border isn't visible anyway.
It's fullscreen mode is actually borderless fullscreen. I noticed I also get stuttering in Windows menus and in browsers occasionally. So my uneducated guess is it's something to do with old GPUs and the Windows desktop compositor in Win 11.
The way I fixed it was to use Special K to force the game into exclusive fullscreen mode. Also, I enabled the "sleepless render thread" option.
My hunch is that either of those options would fix the issue, but I enabled both to be on the safe side.
One major bonus to all this is that I discovered Special K has an "auto low-latency" mode that significantly reduces input lag by delaying the render thread. As long as you have the GPU overhead, and the game is fairly optimized, it should work flawlessly. I'm going to start using it on all my games on this PC, since even an HD 5450 can 240p games at 100's if not 1,000's of FPS.
If you have one working on the same PC, you can use a more modern GPU to render the game.
GPU power isn't the issue with Steel Assault. I'm pretty sure that the GPU is basically downclocking too much because the game is too
easy to render. This leads to stuttering as GPU utilization jumps between 0% and 15% constantly.
By doing the fixes above, my GPU will stay closer to a steady 10-15%.
On top of that, rendering on a separate GPU typically causes an extra frame of input lag, in my experience. Also, frame pacing can frequently become a problem, which would put me back at square one with this issue.