Like everything else, depends on condition as to worth. Picture would be nice. Anderson's bluebook of model engines may give you an idea since they cover just about all engines and fairly current prices.

Hello Gary,

I like old spark engines, so just pack it up and send it to me! Seriously, Brats have always fascinated me, they are rare birds and I guess they are quite valuable........sigh, so I suppose yours will finish up in some glass case collection. Brats were made in 1938 and 1939, the 1939 ones were around a .15 size and the 1938 ones were a little less capacity. The very last ones had Brat stamped on the bypass. They were made in very small batches with some considerable variation between batches. This is one of the reasons that I find them so interesting, there probably isn't such a thing as a definitive Brat!
  I know that there are some repros about, although they are stamped as such below the mounts. I was entrusted with a non runner for restoration. This was a very early sand cast version from 1937, I didn't think that any existed, but there is at least one! After restoration, I was allowed to fly it. No great shakes power wise, but very easy to handle. I suspect that my fits and finishes may have been better than the originals (boasting!!), so the one I tried may have been better than most.
  You have a rather rare beast, so make sure you do a little old time free flight, or at least sell it to someone that will use it rather than a collector!

Regards,

Andrew.

Here's a photo of my Bluebird 28. Weight of the bare engine is 8.25 ounces plus 1.5 ounces for the muffler.