First off, welcome to the forum!
Now on to business. NEVER use starting fluid on these, or IMO any engine that you want to keep! Ether is very dry and if using too much, can blow the head off, crack a piston, many things.
If your fuel lines are routed correctly, if they are new, you have to crank the engine for 15-20 seconds (remove the plugs, this helps the starter) FYI do not crank the engine over (with plugs in) for longer than 10 second intervals. Then let cool 20-30 seconds before trying to start it again.
As for the carb, since it is apart, you want to use carb cleaner and spray through every single orifice and make sure the fluid comes out somewhere from the carb. Next take a little wire (like from a wire wheel) and poke through all the little passages (especially the idle/mid range circuits. (inside the carb bore, hold the throttle/butterfly valve open and poke through each of the little holes at the top of the carb. Also, make sure to clean the little mesh filter inside the carb. Blow through all the passages again, and re-assemble. If you are using the same needle, seat, and spring combo, I would leave pop off test alone (unless you have a tester, then go ahead and test it. If not, no worries about it) If you are replacing gaskets, use only genuine OEM Mikuni rebuild kits. Not aftermarket.
Try saving the carb before rebuilding. The rusted circle is called a welch plug. To remove it, hammer a nail, or sharp pick through the center, then pry out. Under there, you will see tiny holes for the idle circuit. If the carb had lots of corrosion or water, then its possible there may be some gunk in there. But I would do that as a last resort. Normally, you don't have to mess with those.
