C-wire on Weil McLain cGA Gold has low voltage
Yes you have 24vac at your boiler.   Assuming you have enough conductors in the wire to your thermostat, it can be powered.
How many wires come from your thermostat to the boiler?  The old thermo in the wiring diagram looks to just be a switch on the hot leg of the 24vac, you'll need the neutral side ( c-wire ) run to the thermostat as well.  Where did you pull your c-wire feed from ?
It looks like you've circled something that has a ground, an aquastat, and the outgoing hot leg and incoming hot leg of the 24v.
The wire nut you've circled - I am going to assuming is your new c-wire - has a blue wire and a black wire and I can't tell where either of them go.