The solenoid is to cut fuel flow when you shut the motor off.
The diaphragm works exactly the same way it does on many car carbies eg holden varajet.
Generally its only use is if it is electric or remote start, but most have them anyway. If it had electric start, it would automatically shut the choke valve when cold, once it fires, the vacuum on the diaphragm pulls the choke off
that part is probably a vacuum reducer (many cars used similar with vac operated heater valves) or perhaps a one way to prevent a backfire blowing the diaphragm
If its manual start, block the vacuum hose and forget it
