Change and bleed the clutch fluid, if problem persists, likely that you need new seals in the clutch slave, master, or both.
About the shifter seal, I don't think any disassembly is needed other than removing the shifter from the shaft. Don't quote me, but I think that's the case.
I run a 200/55 too, I doubt that caused any problems for the bike.
If the bike was used for stunting, then it would probably have some radical sprocket out back. OEM is a 40 tooth. What does your's have?
You may want to pick up a fuel pressure regulator too. If the bike's been stored for a while, the diaphragm deteriorates and starts to leak once she's been running for a bit again, making your bike run fat.
Good luck, hope you getter fixed before the weekend!
EDIT: you may want to change the engine oil to a good quality synthetic if you haven't already. Old oil can cause problems too.