Hi Lee, I'm not au fait with your rifle. Looking briefly at images of it, it looks like it may be based on the Gamo platform, as shared with a few other guns originating from China?
Have a look at the Mainspring Data Thread in the Useful Section. Go to the last page. You'll see an excellent post by Phil Russell giving spring stats of many springs. And also an updated one for Titan springs posted by Pete, "look no hands". Alternatively, you could try Tony Wall at SFS with your required dimensions.
If it is based on the Gamo design, take a look on the Welsh Willy site to get an idea of the spring guide design required.
Hope this helps a little and be sure to shout up for any further help.
P.S.....The stock looks nice on the Thunderjet.