You are correct about the plastic cover on the rear of the engine. I have seen gray ones and black ones. That cover missing is not a show-stopper and I think it is there as a safety device for sparking.
The throttle return spring attaches to the hole in the lobe on the throttle mechanism and goes back to attach to a bolt head on the top of the block. The bolt head is about 4 - 6 " behind the lobe, straight back. It is not a stiff spring at all. I am going to shorten mine a bit to get a bit stiffer spring for a crisper return on the idle. I'd take a picture of mine and show you but I'd be a bit shown up by that nice looking engine of yours. Hopefully the internals are as nice as the external. Compression, etc.....
I can only suggest a conversion kit to an electronic ignition. A very good investment.
for your parts
Damnn that looks nice. Good job. I don't see anything that doesn't look original. The newer and smaller starters are top notch also.