what happened? no oil, one piston firing

[udell85]

i bought a 98 gs500e with 10,000 miles on it. i rode it for about a month and the on the freeway it loses power and starts making a strange noise. i pull off asap and kill the motor. i check the oil and there's hardly a 1/2 quart. i checked it the day i bought it and it was perfect. i'm assuming the dude that sold it to me knew something i didn't. i've pulled the motor and found that a piston is really lose. i haven't taken it apart yet other than the head off. the bike shows signs of being screwed with prior to my teardown (weird wire routing, missing screws, ect)

what i need to know is should i do a total rebuild, bottom end rebuild, try buying a used engine or what?

if i do a rebuild what parts will i need, other that a new piston and possibly crank and bearings. i'll post pics as i take it apart. thanks