Old TVs are generally only good for older gaming consoles, 8-bit computers, and some 16-bit computers with low resolution. Don't throw away the TV though, because it's part of history. Lots of people are interested in collecting old TVs. Donate it to a thrift shop if you want to unload it fast.
Theoretically, you could convert any signal to the TV with the right assortment of RF modulator, cables, converters, SCART connectors, etc, but that's expensive and it's easier to stick to systems that naturally output to RF or composite video in 4:3 standard definition.
The problem with a Raspberry PI is it outputs in digital HDMI. You may be able to find an old Raspberry PI that has composite output, but you still need to convert it with an RF modulator. VCRs with RF output contain an RF modulator, so they are frequently used to convert composite to RF.
The second problem is resolution. If you want to do anything with text, you'll have a terrible time reading it. If your goal is gaming or media streaming, those will work.
Those old computers have very limited functionality for modern use. They're essentially only good for games. There is a subculture among the classic gaming community who like to connect their old consoles and 8-bit computers to antique televisions. They will also have lots of knowledge about how to do this.
Two forums I'm a member of could help: