Mate the wobble and the engine problems are NOT linked.
You need to be more specific as to what the bike/engine is doing when you change up, is the engine still revving but the bike is slowing?
If so it could be the clutch is slipping. Is the bike just slowing and the engine is dying as it's revved in which case this could be fuel problem, too rich or weak mixture.
The handling issues again could be alsorts, but from what you say the bike doesn't appear to be in a good state or repair, where did you get it from, where in the world are you? Leaking fork seal won't help but most wobbles are caused by rear end problems, worn shock, seized swinging arm/ shock linkage bearings/bushes and or worn/mismatched tyres.