edit: i mean like the 7-8 digit number in the .toc file
The arena frame code changed from 4.x to 5.x (added the prep-frames and spec icons). It may have also changed from 3.x to 4.x so I'm not certain that any version of sArena will work with 3.3.5.
Regardless, you are right. Changing the interface number in the .toc file to 30300 will allow the addon to be enabled. No promises that it'll actually function, though.