Yup, ^^ What he said ^^
Yes, that power transformer has plenty of current for an extra 12AU7 or 12AX7 tube depending on the gain needed. Since this is a single ended amp, you will only need a single preamp tube.
It's a simple circuit and I think I have a schematic around here somewhere that you could use.
EDIT: Nope, don't have a schematic but any schematic that shows the front end preamp tube for one of those single ended Magnavox stereo amps should work fine. Feedback might have to be tweaked.