Parts Suppliers for A12s

[A12inMG]

For those of you who don't know, I have a Datsun A12 engine and transmission that came in my 1964 MG Midget. I have no idea what car it originally came from and I'm having trouble finding a supplier for parts. I'm mainly just refreshing the engine and transmission, so I need things like new shift bushings, shift pin, valve cover gaskets, points, oil cap, rear transmission seal, etc.
Also, I am missing the flywheel cover plate; any suggestions?
Another question/situation: the stock carburetor is too tall in the MG. What other carb/manifold options are there for the A12 (besides dual carbs) that sit lower?