okay, let's answer some questions-
Anybody know if a J13 fuel pump is a direct bolt-on to the E1?
Yip, no problem. Same actuating arm and an exact fit to the block. Still need the spacer in which you can trace new gaskets from. I'd go with new brass bolts and washer as they do no corrode.
Sorry, down hill from here -
NLA but does anyone have access to a Nikki 2D-30C rebuild kit
Nope, sorry. No longer made and at least not in stock with Classic Garage in Brewster, NY (just north of Syracuse) Somewhere I have the reject email from them telling me that Royz no longer makes them. So you will need to make your own main body gaskets and source the O-rings from another kit. Occasionally one comes up on eBay, usually from Australia. PM me if you need the leather boot for the accelerator pump.
Lastly, are J13 Hitachi carbs a direct bolt-on to the E1?
No they are NOT an exact fit. The J13 the preheat on the J13 carb to exhaust manifold is ~1/2 longer to the will not match up.
Ohh, forgot. if your 320 is still POSITIVE Ground be careful with a electric fuel pump as most are wired for NEGATIVE Ground autos. It might work at first, but it will eventually fail at the most dangerous or embarrassing place on the road. Also, electric fuel pumps are meant to push, not pull. So read the direction for where the manufacturer says to place the electric fuel pump. Probably something along the lines of - "within 12-18 inches of the fuel tank and below the center line of the fuel tank"
Might check the Running on a budget thread for more information and pics.