Ian has the right idea: first see if you can repair the old supply. Determine where the smoke and sparks came from, and see if you can replace the parts involved. If not, you may have to build a new supply. You generally can't just series the outputs from computer supplies, since most of the outputs will share a common ground (even if there are separate leads). These supplies are rather peculiar to the equipment, so replacement can be tricky. They usually provide some kind of voltage regulation, but not always. They usually have peculiar wiring and switching schemes that you have to figure out properly to make the supply work. Building can be an expensive and complicated option. Try repair.

Another possibility would be to replace the bulb in the Vivatar with a different kind that used 12 or 24 volts (with 80W power). You can find regulated supplies for these voltages easily. That assumes that the enlarger is wired in some simple manner and that you do not have to power a fan. If there is a fan, you may need to replace that, too.