My antenna broke on my sr300 I noticed it is not a coax so would I be able to use wire from a fm receiver?

I believe so, yes. I had to replace a couple of antenna wires on my Spektrum receivers recently, & best I could tell, they're just basic 24AWG wire(which I have easy access to at work, we use it in all colors all the time). I haven't had a chance to test them on a track yet, but I played with on in my VTA car in the street, & I couldn't tell that I had any glitching, & I ran it as far away from me as I could see safely, so I think it's working fine(though I'll be more sure after this next weekend when I can test it at the track)....

Any wire with the same length and same diameter would fit perfect!

Can anyone confirm or deny these claims?

These comments are correct. There's nothing special about the wire. Use a similar gauge, the same length as the original.