So...

Is there any way for me to wire a 1/4 jack to a synth so that when I play my guitar, it plays synth sounds? I'm looking into starting to make my own effects pedals, and I just need to find out how to do stuff.
I have capacitors, but no transistors to build a distortion pedal.
Does an old 90's Yamaha Keyboard have the equipment for me to make a distortion pedal? (Or, rather, just a distortion box) If not, is there a way for me to wire leads into the keyboard so that it processes my guitar's inputs?

Do you want to control a synth, or do you want the guitar to only sound like a synth? Furthermore, unless you are an engieering prodigy you can't do that yourself. Out of curiosity, what keyboard do you have?

Also, Google "Roland GR-20", I have one and it might be what you are looking for.