I've got a GPX250 and recently the chain came off and it spat the front sprocket. I've replaced the front sprocket, plate & bolts, the chain & adjusted everything up. Now, I can only change from neutral up into second and back again. I can't get it down into first, it just won't budge, and it won't change up any higher than 3rd. I'm guessing the chain mishap has caused some damage to the gearbox. Is there anything I can check that's not too tricky? I'm not the most mechnically minded, and I live in a very remote area, so there're no bike mechanics for a few hunderd kms!