You can just use a 19" television too. I hooked one up to my Midway Clowns (same basic hardware as Space Invaders) when it was giving me trouble and it looked great.
I put a 19" color monitor into my space invaders and wired it up for black and white, but they will not fit without cutting the wood monitor frame slightly larger. Also, the neck is longer on them so you have to remove the wood where the chassis normally sits on or you'll bust the neck (like I did the first time) ouch......
It is out in the shed now. I bought two of them at goodwill and tried them both. Both held their input setting after being shut down. That is something you can try at the thrift store before you even buy the TV set.
If you're planning to come near Philly anytime soon I can rejuv it for ya. Keep the b&w alive!
Also note that I think there are in-line resistors dangling off the picture tube socket. They can go bad and cause a dim picture as well I believe (Dont quote me on this)