Can I use the Y wire in place of the C wire for my new thermostat?
Yes, you can swap out and reuse the Y wire as a C wire, since you don't have an air conditioner.   Or use the capped off wire (that's probably better).

hey're just copper wires, and there's no firm color code anyway. Take photos before you disconnect anything, and consider leaving a note tucked inside the thermostat documenting whatever you came up with.

Check this excellent answer for diagrams:
https://diy.stackexchange.com/a/10527/5960

Consider posting photos to SE when asking questions.