Not hard, but you might like the SD-1 changes better. The SD-1 has asymmetrical clipping (changes harmonic content, a lot of people think it sounds more 'tube-like' to have that kind of clipping) and some other changes. You can mod a TS-7 to 808 specs - it's a TS-9 with a different sticker and clicky knobs.
For a first build, try Aron Nelson's Smash Drive - interesting smash-the-fuck-out-of-an-op-amp circuit and it's easy enough to get you used to the way things work but good-sounding enough to be on your pedal board for a while if you like goofy smashed distortion. Link is at
http://www.generalguitargadgets.com/diagrams/smashdrive_lo.gif for a PCB, you can just use wires and perfboard to get the same stuff done if you don't want to get into PCB etching.
You'll also be interested in guitar preamps - whether you're going for gain or a lower impedance for driving pedals, you can find something neat that'll fit in some cavity somewhere. Hell, Matt Bellamy has fuzz boxes built into his guitars, give it all a try. Preamps are also good ways to figure out how simple circuits work, the intro project at diystompboxes.com/smfforum is a basic preamp that'll help you get started if the Smash Drive is too much at first.
The most helpful thing to do if you don't want to jump into building from scratch is to start taking out resistors and putting pots in their place - granted, with a DS-1 or an SD-1 you'll want to know which ones do what before soldering in twenty potentiometers, but doing that can tell you a lot about what does what in a circuit.
If you have any questions, PM me and I'll start throwing around links - be careful, though, it's addictive as crack on crack.
Jeff
