You can get new bearings. Your mechanic should have not problem in matching an old to new.
http://forums.pelicanparts.com/porsche-924-944-968-technical-forum/767617-what-correct-alternator-bearing.html
The normal part replaced is the regulator.
The part that is usually the most forgotton is the "slip ring".
The old regulator will wear a groove and the new regulator brushes will not have a flat surface to contact with.
Add all the parts together and just get a "remanufactured" --most parts replaced rather than a "rebuilt" --cleaned, painted, tested---parts replaced are minimal.
90 or 115amp?
I would go with the 115 amp if the same price in the UK. The wires on your '84 have built a resistance. You should actually change the battery
"+ -" wires and the battery lead connectors.
Many go with the "
Ice Shark battery cables or similiar. Top of the line.
IceShark Cable Kits... - Rennlist Discussion Forums
GL
J_AZ