If you have the knowhow, standard american TV sets CAN be converted to arcade monitors.
Basically all you need to do in pinpoint the 4 spots on the board where RF becomes composite and composite becomes s-video (so you can inject the sync signal there), and then were the s-video gets broken down into the separate RGB or in many cases Component Cr Cb Y signal before it gets amped and sent to the neckboard of the tube.
Just cut the traces prior to that point to not get noise, of course toss on a isolation transformer and inject your video.
Now, do you want to do this? Probably not. Takes a fair amount of digging and get it wrong and you may hurt the chassis, your game board or yourself. Also all of these TVs likely need a good cap kit already and there are no standard kits for them, and there is no documentation readily available.
This project HAS been done before--there was a thread on BYOAC that was censored after someone copying it hurt themselves or something but it had been successful. Myself, I've taken a commodore S-video / Digital RGB monitor (Think 16 level of color IBM PC CGA) and hacked out the Digital->Analog converter and inected analog RGB successfully and got a fully working test bench monitor from it.
You're not going to find a lot of support but it's possible.
However, in the end, it's easier to pick up a replacement chassis for $60 and plug it into the existing tube+yoke (no yoke swap) if you really want a 'fresh' monitor. But given the wide range of yokes, sometimes geometry aren't quite right and many time the replacement chassis have weak power supplies and give hiccups as well.
If you're in Europe or can find some industrial US TV sets with SCART connectors... then it's super easy--the SCART connector has all the pins needed to directly plug in analog CGA and be done.. all from the safety of the outside of the case.