You seem to have no idea about engines and mechanics, not being critical when i say that, youre in the majority. The info youve given isnt precise enough, when you say its been leaking"engine oil".... where from? is it gearbox oil you mean or have you been running it with no two stroke oil?
Either way though, from the info youve given, i would hazard a guess that it is going to be an engine out full strip down. but first maybe the head and barrel can be taken off to see if the damage is there. I think its not a job for a complete novice though...even one who can follow instructions, you need to rope someone in to help you, or pay dearly to have it fixed. Where are you by the way?
If you do take it in, you will need a quote and only you can decide whether its worth fixing, have you got access to a van?