Intellivision:Yannick RGB Mod
One option for RGB output on an Intellivision is to use the Yannick RGB board. Note that you will need to either cut a hole in the case or remove the RF port completely to be able to access the video port you wish to install. Audio output is also mono, so you may wish to bridge two pins of the output jack or to add extra RCA jacks to be able to send audio to two speakers.
Ivory Tower Collections created a great installation guide as well the below video demonstrating the installation of the board with some extra information.
V2.0 Sync Issues Fix
Version 2.0 of the board was created to fix sync issues with some NTSC TVs. However, in some cases after installing the Yannick RGB mod, you may see that a top portion of the screen is misaligned or "flagging" as shown in the image below.
If this is the case with your install, you can fix this by cutting a trace and bridging one side of the trace to ground on the bottom of the RGB board as described below, which effectively turns it into a version 1.0 board.