Anyone here a sound engineer or having experience in mastering who could give me some advice on how to remaster low quality audio files (perhaps using audacity and it's billions of filters), as in adding back in the low and high end that was cut off, to a certain point?

So far I've tried an exciter followed up with a compressor in Audacity but my understanding of how these things work is rather...limited.

ps: don't tell me to re-rip as this is not possible due to being a low quality internet mp3 release.

It's possible, but not easy.

Basically you have to recreate those frequencies. You can to some extent by running it through stereo and reverb filters.

In the end though, there is only so much you can do with bad input.

First experiment with reverb filters first, then go to equalizers, etc. It's a pretty time consuming process, but not impossible.

aren't there Bicubic & cosine filters?

and a lot of rinse and repeat... gives me a headache

I'd just hire a mastering studio if I was like richie rich

most likely.. how do they bring back lost frequecies?

FFT + interpolation

Similar to image upscaling, just much better results as sounds are much better harmonic and so are FFT-able.