i too had this problem but not quite as serious,
i wanted to get into the world of trials and i had already rode dirt bikes, so i went and found a gas gas jtr on ebay for 540 quid, knowing this was a bargain i went for it, went to go pick it up and it was at some guys junkyard but it seemed to run okay, i took it home and took it out for a proper ride and the back brake didnt work it was seized open. the clutch was dodgy and it was impossible to get into gear, half of the plastics were missing and the exhaust held up with silicone sealant. only little problems so i bought new plastics a new brake setup and give it a good strip down fresh fuel and a new spark plug, now it runs fine and gears seem to be okay everynow and again it skips a gear, but its okay as a beginners bike. serves me well, however soon ill be interesting in renewing into a newer lighter model, hit me up if your interested?