For Carburetor part # 16310-08103 or # 16036-08100 these are the Assembly-Jet, main air bleed primary and secondary. So do not know what the power bar jet is. Also, do not know where to purchase these seperately.
For the Thermal Transmitter 25250-30100 (AKA a thermostat) this is available from NAPA or eBay. Just watch the threads as one type is fine and the other is coarse.
Brakes are easy. Brake and Clutch rebuild kits are available on eBay or a complete unit order through your parts store, you will need to tell them its for a 311 Fairlady or 520. Look at picture first on their screen. Also make sure its SAE and not metric on the 520 for the brake line fitting. The brake MC is 3/4 bore and the clutch is at 5/8 in. All brake wheel cylinders are 3/4 inch bore. You can occassionally find the rear wheel cylinders complete for around $18-$25 on eBay. But I have only seen the front once or twice and I snagged them both times. 3/4 in front kits are available for the 620 on eBay, you just have to re-use your boot/cover as the one is the kit will not work.
Part number for the drums are 406 and 405 by BrakeBest and I think BorgWarner has the same number 406 and 405 or 406a and 405a. So no need to buy these on eBay.