A couple of things to understand about the Compaq Portables video card. The default CGA card from Compaq was able to display CGA resolution in graphics modes, but higher res MDA style text modes. The built-in CRT was dual mode and able to switch between the two timings. If you were able to acquire the unicorn Compaq EGA card, it could also display to the built-in CRT - even in EGA resolutions.
If you decide to convert to an LCD screen, you will lose the ability to display the dual-modes from the CGA card, and have to limit yourself to the standard CGA resolutions. You will need an LCD panel capable of NTSC input, which will look pretty bad in 80 column mode, or get one of those NTSC -> VGA converters (usually a chain of them to fully convert digital RGB from the CGA card to VGA/HDMI ouput). Or, if you have the EGA, a slightly shorter chain of converters to get to VGA output.
Really, the Compaq Portable has one of the most under-appreciated video subsystems of any of the PC/XT clones and you probably wouldn't get the same quality from an LCD conversion.