Scales and modes are a guidepoint, but if you're going to sit down and write a solo, as opposed to improvising one (which is what your friend jimmy page mostly did) then I would suggest you think more about melody. Come up with an idea that interests you.
I find that once you teach somebody who likes rock how to play the pentatonic scale, and they become fluent with it, they start to think that going up and down the scale stepwise, and playing triplet licks constitutes a solo. Remember, the key is to be interesting with music, not just to play the notes that are in the right key as fast as you can.
If a song is basically a blues form (like most early zep songs) and is in the key of A or E (like all early zep songs) then you'll be safe improvising with the A or E minor pentatonic scales, respectively.