need a slow scan converter
I need a fast scan to slow scan video adapter and a slow scan adapter to connect between my rig and a monitor.
would also consider using a computer card to take fast scan, convert to slow scan then send it to the tx and then from the rx convert to a computer screen image.
maybe an old robot unit.
It would be simpler to use a computer compatible camera connect to USB port, capture an image while saving it as a file, then use common SSTV software to manipulate the image if needed, and transmit it.
You could perhaps pick up a used USB webcam at a yard sale, craigs list, or thrift store for cheap, or spend as much as you want on Nikon, Olympus, etc.