There were some details, that prevents me to threat this very interesting and well-minded concept as useful for construction.
1) very small wings and forward horizontal stabilizer - even with canard layout they, IMHO, too small and without proper flaps couldn't provide good take-off and landing. Note, that size of wings and empennage and it's area didn't depicted on drawings?
2) 24-cylinder engine - very provisional in terms of size, weight, power - and necessary size of cooling system
3) undercarriage layout - looks that angle between main wheel and propeller down point is too small, as a ground clearance
4) pilot accommodation is too tight and required some serious safety measures (propellers' drop-off or ejection seat) to bail-out
5) very small size of fuel tanks
However, this design looks to me very advanced, as a provision for future jets: tricycle gear, canard-layout, power plant behind the pilot (providing both good forward view and some 'armor"), sideway channels for airstream.
Some of these features resemble me Heinkel He-178, Gloster E.28/39 - and SAAB twin-boom fighters!
I'm curios, if there any detailed story of J-21 development? And was Sason somehow connected with it?