Can a composite video line doubler be made using simple hardware components? (composite -> progressive scan)
Interlaced scan composite video signal isn't easily convertable to similar progressive-scan signal. You need to store the whole frame into some kind of memory and resend it at different speed using progressive scan. The simpliest way you will need a DAC-ADC pair with Fq about 6.5 MHz, an SRAM or ADRAM unit capable of running at 13 MHz clock and holding two frames, and the logic to control both input and output address. This way you will be able to store two interlaced half-frames and resend them back as progressively-scanned frames.

Everything said above applies to monochrome images as I don't know how any color-coding system works with progressive scan.

The image produced is likely to have visible interlacing artifacts, especially if there are horizontally moving objects in the image. It's a quite difficult task to remove them, but if you really want to reduce them, the better option is to have a PC with a analog video capture card and a program that processes and displays the image captured.