There's not alot to do in the bottom half,besides the crank.At least inspect the crank!!!bearings and clearnces.So far i've been running a new yamaha crank on a modded motor, it's 6 years old. all I did one year was replace one of the bearings.never had it trued and welded,But the next time it's torn down to re-ring I'm doing somethingwith the crank!!
Check the tranny out.Some say if ya speed shift or don't use the clutch alot, you can wear out the dogs on the gears.I had to replace a couple of gears before.