I have actually changed the roller assembly in a roller tappet assembly. The only real trick is that the bearing axle is usually held in place by swaging (if that is the correct term) the body of the follower. So after you drive out the axle you must find a way of reswaging or staking the body of the follower to securely locate the axle. I was working with Oz Ford SOHC straight six rollers and the correct parts were available from Ina. I have vague idea that a lot of roller followers are special sizes and probably not easily available.
