How long do you want to own the bike? If you plan on riding/owning it for awhile. I'd replace them all. You can rent a bearing puller from an auto parts store or just buy one from Sears. If I have the cases split, I always replace the bearings. Just cause they are there and easy to reach while the crank is split. I try to avoid splitting the cases more than once a year. The sludge can come from alot of stuff, like the clutch, gear wear, dirt coming through the oil plug. It looks like years of buildup.