Well, the makvision isn't dead until it's dead, it might last for years, but i wouldn't throw away the Toshiba. But if you want to stick with CRT, as i personally would go to a lot of effort to, you might think about:
1. Getting a decent 1:1 SCART-to-component converter to feed the toshiba, and move the toshiba into the cabinet. If it has component inputs. If not, think about getting a TV with SCART or compoent inputs, that way you get the best quality image. And anything that takes component will take S-Video for the C64 and composite or RF for whatever else.
2. Hacking RGB into the Toshiba, then using S-Video and composite and RF as above. There's a good thread on RGB hacking on the Shmups forums, i'd recommend reading the first page, then going to the end of the thread and working backwards until you understand enough to start you looking for the information you need.
3. Hunting down a 20" PVM.
4. Getting a decent used arcade monitor, preferably a Nanao, though this still leaves the problem of getting any signal other than RGBS into it.