It should be noted that new projection CRTs are available fairly inexpensively.
My personal opinion: swapping out projection monitors with LCD/plasma looks horrible. They are typically haphazardly installed and the aspect ratio is wrong. For a collector with limited space though, I understand this might make good sense.
You might think it makes sense from a reliability standpoint for operators to do this, but I'd almost certainly bet that rebuilding a projector with new tubes & caps will outlast any tv retrofit and cost less, though it is more work.
Since nobody makes or even wants rear-projection sets anymore, replacement CRTs are inexpensive. For Sega's Mitsubishi, Toshiba & Hitachi projectors, VDC (
http://www.videodisplay.com/ they also own what used to be penn-tran/wintron/lexel), who made tubes for a lot of projectors, has tubes for just about anything for around $99 each, and IIRC that includes the service of swapping out anything attached to the tube.. you just send them your burned CRTs with yokes & all (projection sets might have 2 or 3 yokes installed on each tube), and you'll get back new tubes you can just plug back in. Otherwise you can often find new tubes on ebay & elsewhere for $50 or less. If you have a US-build Namco game: they used off-the-shelf mits TVs, so just find any old mits model VS-50* from CL for free-$50 and you can drop it in. Just about any home-use projection set should have decent CRTs, or at least nothing more than a channel ID burned into a corner.
Personally, I'd rather spend $0-$400 to actually "restore" a game and keep it original. I have a few projection games I'm going to be re-tubing within the next couple months.
If you have burned projection CRTs just google the tube number or search VDC's site & ebay. Many tubes of a similar class can be swapped around as well, just ask VDC if they don't have the exact replacement.
There is a learning curve in working on a projection set I suppose, mainly in having to converge new tubes and setting your G2 (screen control) & color balancing, so it isn't exactly for beginners. Nearly all CRT projectors (including cheapo consumer sets) have self-discharging bleed-down resistors. You don't have to discharge a projection set or even remove the anode from the tube, you just remove the anode lead from the HV splitter block/flyback with a quick-disconnect.. They're pretty easy to work on, really.