For anyone who likes their MiSTer on a VGA CRT @ 240p/120Hz and would like to play R-Type at the correct framerate (55Hz), I've just tested a custom modeline (2048 x 256 @ 110Hz), which works perfectly.
video_mode=2048,48,208,256,256,1,3,14,77160
vga_scaler=1
vscale_mode=1
You can also add the following to your mister.ini (for this game or any version of):
[rtype*]
video_mode=2048,48,208,256,256,1,3,14,77160