Well, I went ahead and rebuilt my old raw water pump just to see if I could. I have it back on the boat and it runs fine w/no leaks.
The tricky bit was pushing the driveshaft and bearings out of the pump housing without damaging anything. When Johnson builds these pumps they make sure the bearings stay in the pump housing by hammer punching the bearing housing OD in one spot. I drilled that hammered spot out to help relieve the pressure and the bearings and shaft came out without much trouble.
I found the SKF brand bearings and an near-OEM equivalent water seal at an online bearing supply house. My total parts cost was about $20. If you are willing to use Chinese no-name brand bearings the parts cost can be reduced to about $10.
To retain the bearings I drilled and tapped three holes around the bearing housing for very short 8-32 setscrews. Now I have a pump I can easily rebuild the next time.
So you can rebuild your raw water pump yourself if you are so inclined. Most people will still probably just buy a new replacement, but where's the fun in that?