Does anyone know who manufactured the original starter for a 1980 RX-7 and what is the best replacement if one were going to begin a restoration?

 

Original manufacturer is Mitsubishi Electric co, same with the Alternators. As far as replacements are concerned, I've not had good results with ones from the local Autozone/Advance Auto/Oreillys as of yet. Granted, I am turning over 50% more engine than stock (I have a 20B now), but a new starter from the parts store should NOT imitate Lassie when you turn the key. Have you considered checking locally for a shop that rebuilds starters/alternators? It would most likely yield a better product than one 'rebuilt' in China/Mexico/Alpha Centauri

Another option would be to send it to Banzai Racing for a 2.0 KW conversion as seen here.
High Torque 2KW Starter (87-91 Turbo II RX-7)

While they do not advertise this upgrade for SA/FBs, it stands to reason that it would be the same overall modification (IE transplanting certain parts from an Automatic's starter to a manual transmission's starter). After all, automatic RX-7's starters were all rated at 2.0 KW. Try asking about it, I'd trust Chris at Banzai Racing more than anyone else to rebuild/upgrade a starter.