I bought some old bowling balls from the  thrift store recently and they didn't perform like they would on the first note I used them. What could I do to help them perform like the used to?

The usual resurfacing and de-oiling, then finding the right surface to use on them for the conditions at hand.

My approach has been to take the ball in question and sand down with a course grit (in a way to maintain or improve roundness)....Haus or similar machine, CAB blocks, or at a minimum being careful with a spinner.  I like using something pretty course ~320 grit or so to remover a lot of the scratches, nicks, etc. and expose fresh surface.  I then go through some type of deoiling process...all sorts of posts on here about it.  Once complete and no more is coming out, I go back to the spinner, hit it with 320 grit again quickly, then start stepping up grits towards the desired final grit.  I usually stop one grit below the original surface finish of the ball.  I've only ever had one instance of where thus just didn't work...  Overall I've had great success.

This.  You may even can just do it without deoiling first.

What a lot of people don't realize that a lot of the current performance of coverstocks is the surface "tricks" the manufacturers are doing now.  Having a very rough first grit and then immediately jumping to high numbers (i.e. 500 then a light 2000 or 4000).   I remember when I was on staff with Roto and I polished a Cell and wanted to get it back to factory reaction.  Couldn't get it to react close to what it had box and there was very little games on it.  Found out a it was 360grit then a light 2000.  I was doing the old 500/800/1000/2000 routine which made it too smooth.  Doing 360 then light 2000 restored it to factory. I also started doing it to "weaker" balls like a Mars and it was amazing the reaction difference.

Take an older ball and do the same.  I'm still using a Nomad, Thunderstruck Pearl and a Virtual Gravity.  Before I started using them again (they were my wife's so had a significant number of games on them), I gave them a "modern" surface touch of 500/2000 VG, 500/4000 Nomad, 500/2000/Polish TS and it's amazing how well they roll.  I plan on doing a similar thing to an old El Nino 2000.