I have tried to capture the elements of both Free as a band, and Kossoff as a player in our backing track. My Paul Kossoff style guitar solo is improvised using Blues guitar licks that Paul Kossoff and other players of the 70’s Rock era employed, but also using Paul's trademark quick urgent finger vibrato, which hopefully makes the whole thing more specific to his sound. As a note about 'that' vibrato, it seems to me that Paul Kossoff dramatically changed his finger vibrato post Free. When you listen to his playing on 'Back Street Crawler' it is much slower and a little wider than the vibrato he was using in his Free years. Was it a work in progress? Was it the chemicals in his system changing his approach? We will never know, but I associate the Free years vibrato as the sound we would recognise as Paul Kossoff. Would you also agree with me that Angus Young from AC/DC has huge elements of Kossoff's vibrato technique in his playing?
As far as tone goes, well we know it was all about his '59 Les Paul and point to point wired Marshalls from the day. I used one of my Les Pauls and a more modern Marshall pushed a little bit at the front end with an old Boss CS2 compressor. After Free split up, Paul Kossoff employed his Strats more, and even a few pedals of the day to push his amps at the front end. But due to his sad death, we will all probably associate the sound he made with a Les Paul more than a Strat, which is why I chose one as the guitar to do this style analysis with.
If you are reaching for the Kossoff sound and a vintage Les Paul/Marshall set-up is out of you price range (and who can afford that sort of gear these days?!) don't despair. For surprisingly little money you can pick up a really good guitar in the 'sort of' Les Paul mode, like the Vintage V100 for silly money, which the GI team still rate as one of the finest value for money guitars on the market today, and if you can't find one of those there are plenty of Epiphone Les Pauls around. Ideally, you are going to want a valve/tube amp to get the sound and in a perfect world that would mean a Marshall, but most good valve amps driven hard will get you somewhere close.
I hope you like the backing track. You need to be on top of your E minor pentatonics for the A section and be able to mix up your A minor and A major pentatonic shapes for the B section. But if you really want to sound like Paul Kossoff, think about that vibrato, and those smaller inverted power chords he employed. Oh, and don't forget tone, feel, soul, phrasing, expression, string bending, dynamics, guitar parts, songwriting, rhythm playing...... need I go on!