Do you know of any sources for larger needle valves? I'm not actually making an aquarium, so I think a higher flow rate would probably be okay
You need to give more details.
In aquariums CO2 flow is extremely low most having 1 to 3 degrees of taper and small orifices and flows like 10-50 ml gas/ minute.
There are needle/ metering valves in all sizes as well as rotometers.
Looks like Radnors are rebranded Victors.
You can remove the outlet end, should be 1/4 npt on the body, put a 1/4 nipple on a female coupler and a flowmeter.
Not sure what 60gph translate into gas terms ( most can handle both) but it's only about $45 at the e of bay.
King Instrument 7530 Series Flowmeter, 60 GPH, 1/4" NPT Fitting