So the cap was shorted. Good to know that you found the problem.
So let's see, have you reinstalled all of the original transistors again? Is the amp keeping fairly stable and not going into failure mode? How does the amp sound?
If the amp is sounding good and staying stable, I'd just replace the bad cap and call it good. The cap is a ceramic cap 47 pf (picofarad) and rated at 100 volts or higher.
The fluctuating 1/4 volt on the output is fairly normal and may go away or lessen once C7 is replaced.