You need more than just the 9N-10000-A generator; that was a specific "B" Circuit Design unit which also needed the specific 9N-10505-A Voltage Regulator. The unit is a 2-Wire/2-Brush, 7 AMP Output, "B" Circuit Design generator with a small 3-5/8" barrel and 3-11/16" keyed pulley but not 'faster' as you said -don't know who told you that. The unit DOES NOT USE the roundcan cutout so if you don't have the correct genny with VR, you may have to settle for a 9N-10000-B or 9N-10000-C unit. 'B' and 'C' units were "A" circuit design with the cutout. The 9N-10000-B generator was also a small 3-5/8" barrel & now a 3-11/16" threaded pulley but with a 1-Wire/3-Brush unit with the 3rd Brush Adjustment via a "HI-LO" slider button on rear of unit. The 'A' and 'B' generators are rare puppies to find. DO NOT CONFUSE nomenclature. The part number suffixes 'A' and 'B' are not the same as the "A" and "B" Circuit Designs; two different animals. I may have an early Generator - will have to get over to barn and look thru my inventory and I do have some NOS Cast Iron Keyed Pulleys used only on the 9N-10000-A unit. NOTE: The 9N-10505-A Voltage Regulator also is even rarer to find, BUT, one can use the FAG-10505 VR with some mounting modifications as both are used with the "B" Circuit Design generators. FORD went back to the "B" Circuit Generator starting with the NAA Model. It is important to know which circuit your genny has so when it needs to be polarized, you do it right. The wrong way will result in letting all the smoke out of the VR. In addition to the GEN and VR, the early 1939 9N had a 20/20 A AMMETER, used only on the first 4000 units 30A Induction unit after; double ribbed fenders; aluminum hood on first 800 units only; horizontal aluminum grille; 4-spoke steering wheel; chromed shift lever ball; ignition ON lamp on dash; key switch on dash; snap-in battery cover; smooth rear axle hubs; non-pressurized radiator; chrome Stant radiator cap; deep throat manifold; and more. An exact-as-original wiring harness can be found at Agri-Services. I also wrote an article on the early 9N generators but you will have to email for a copy if you want all the correct details - admins here won't let me post it.
FORD EARLY 9N GENERATOR & VOLTAGE REGULATOR: