Generally, with that mileage you'll find: intake valve's seats are slightly worn, guides will be in tolerance, rings will be worn, but the piston ring lands may be out of spec.
The valves are stainless, you can get them ground if the wear is not too bad. May cost you $30. Do them all.
Find a shop with a Neway Seat cutter and have them touch up the seats. May take one hour $80.
Do NOT lap the valves. That will ruin the seat shape and you'll get fast wear. The Neway system leaves the surface rough and it gets peened into the perfect shape when the engine is started.
Chemically clean the pistons then measure them. New rings are a bitch to break in on a chrome bore. Do not hone a chrome bore. I've use only fine scotch brite pads to wipe a little texture on the cylinder. Don't put any oil on the bore when you assemble, just a little on the skirt of the piston. This get the ring break-in started quicker.
Why do you think the shift drum needs replacing?
I don't see why you'll need to split the cases. This plain bearing engine will go many more miles than a roller bearing engine. It needs more oil, but it can tolerate more dirt. If you do split the cases, replace the transmission bearings, esp. the output bearing behind the CS sprocket and the bearing behind the clutch. These go first and aren't expensive.
Buy a full engine gasket kit. Way worth the money.
"Dad, can I get a motorcycle when I grow up?"
"Son, you can't do both"