The best trick I've found for installing bearings in anything, is heat and cold. Before you get ready to install the new bearings, put the bearings in the freezer for an hour or two, and either leave the motor coil (or whatever is receiving the bearings, this works for putting bearings in anything) out in the sun if its hot enough where you live, or put it in the oven on the lowest temp for a minute or two (Keep an eye on it!!!). I know the oven part sounds shady, but its fine as long as you use some common sense, you just need it warm, you don't need it scalding how. You want it somewhere in between warm, but not so hot it burns you. At that point the bearings will usually just fall right into place without even needing to be pressed in. Same when re-installing the shaft if it doesn't want to go, put the bell with the shaft on it in the freezer and heat up the coil side with the bearings in just a bit, the shaft will usually then slide right in without much effort. The best thing about this method is ZERO stress on the bearings, it's very easy to damage new bearings when installing them if you press on the inner race with too much force, or they get misaligned while installing them. If that happens, you were probably better off with the original worn bearings than the ones you just damaged trying to replace them. Any heat source will work, just avoid anything with an open flame, I'm certain someone will come along behind me and have a better method for heating up the coil side, as you do want to make sure not to melt the coating on the winding's...
DISCLAIMER: THIS IS JUST A SUGGESTION AND WHAT WORKS FOR ME, I TAKE NO RESPONSIBILITY IF YOU MELT YOUR MOTOR TRYING THIS. THIS REQUIRES SOME COMMON SENSE, YOU DON'T WANT TO COOK YOUR MOTOR AT 400deg LIKE YOUR BAKING A CAKE!!