Processing Time and Pitch of a Stereo Mix

I just found a 25 year old box of cassettes! All demo's I recorded when I was a teenager. I'm very excited.
 
The problem is that they were recorded on a four track deck which recorded at double speed. So now I have the tracks inside Sonar, but they're twice as long as they should be and an octave low.
 
I've used the "Process Length" and "Process Transpose" offline editing tools which did what I needed but the quality isn't great. This MAY be due to the tapes sitting for 25 years lol. I just can't say for sure.
 
Is there a better way to process the time and pitch than what I'm using? Maybe a third party VST or even a completely different program altogether?
 
Once I have the best quality "stems" I can get I'll master them in Sonar.
 
Thanks for any help! These tapes are treasure to me.
 
Gavin

It might be worth trying setting up a project in Sonar or Audacity (or whatever ) at double whatever the sampling frequency of the audio is then importing the audio files into it but leaving them at their original bitrate.

Probably won't sound too good but doubling speed and pitch by any method generally doesn't.

Your best option might be to see if you can get a compatible old 4-track cassette machine off Ebay or similar.

Edit - just remembered (thanks Mettelus) that Audacity has an algorithm that can double speed and pitch, but in my experience, the time I tried it it was to make a much less drastic change (about 25%to speed only) and it left a lot to be desired.

Use the varispeed trick I've described using the Loop Construction window (which in this application has nothing to do with loops). Pitching up an octave will double the frequency and halve the length...exactly what you want.