If the oil is actually coming from the meeting line of the upper and lower engine cases (and not from the adjacent rear main seal that the transmission output shaft goes through), that is a BIG problem, properly solvable only by removing the engine from the bike, splitting the crankcases, cleaning everything up, and then re-applying sealant correctly. Given the significant amount of labour and that the bike is out of warranty ... you might get away with cleaning up the area very thoroughly (the sprocket will have to come off to get proper access to the area) and externally applying a small bead of oil-resistant RTV sealant over the crankcase split. There is about a 90% chance that this won't work but a 10% chance of it working is better than the labour involved in pulling the engine and splitting the crankcases ...
How much leakage are we actually talking about here?
IF this leak really is coming from the crankcase mating line, OR from the adjacent rear main seal ... either way, the leak is above the normal static oil level (engine not running) so it will not leak except while the engine is running.
This may be something that you just ignore. One of my other bikes runs well but it is an oil burner (it has over 110,000 km on it). Could it be fixed by overhauling the engine? Sure. Do I want to spend the money to do that on a bike that is essentially worthless? Absolutely not. The fix? Check the oil level every 1000 km and top it up. Oil is cheaper than a rebuild. Oil is cheaper than a new bike. If and when it actually blows up, that's when I'll buy a new bike.
If the oil leak is significant enough to represent a safety hazard due to oil getting on the rear tire, of course that is a different story. A drip every few minutes and only while the engine is running isn't going to do that.