Digging through Google search results for "Middle Stick" gave me:
The Graupner Middle Stick was used to introduce the .30 cubic inch rotary engine in 1970.
Another page lists the weight as 5 pounds and wing span 55 inches, so a regular .40 class glow engine should work on it.

40 size kits (currently in production) tend to cost between $50 and $130. I can't guess what your kit is worth. It is somewhat rare, possibly very rare, but that may or may not translate to a high price.