I have 2 fox engines that look just like Yours, one is a .29bb and the other is a .36bb
I run mine on 10% nitro and 24% all castor oil.
The 29 had a 29 on the side of the mount flange,
The .36 didn't have any markings so I pulled the head and button and measured the bore & stroke to find out which one I had.
Both are great running engines, but any hard landings and You'll end up using a strap type muffler.

I also have the same style fox 40bb that has the 2 bearing crank ( New never run.)

Jeff [8D]