Dragstar
Without wishing to sound too harsh, but your lack of knowledge / maintenance of a pretty much indestructible engine caused this dilemma. Bearing in mind you didn't know how much oil was in the engine, you knew it had an air leak so ran on choke to compensate instead of fixing the fault and the most basic piece of maintenance i.e. Chain adjustment , you ignored to the extend that it came off the sprocket and potentially wrecked your engine tells me that you should just concentrate on fixing the bike properly and look after it until you pass your test (if you haven't already) and never mind any big bore kits that others have advised may only cause you more hassle
It may sound harsh but I dont mean to, just got you and your bikes best interest at heart