As Rex has asked, have you had the bike running since you,ve owned it, or did it start doing this straight away. If it started straight away you should have taken it back to whoever you bought it from, if, as you say, he is a professional, he should have fixed it for you free of charge. However, since you have done some work yourself, he may not be willing to touch it now.
You say you changed the ratchet gear, why? was it worn or damaged. Whatever, you are going to have to strip it down again to check what you may have done wrong, I take it your using a workshop manual for this. You may have assembled the clutch lever in the gearbox wrongly somehow causing the clutch to be held off.