There is this guy who has made some similar engine prototypes in this field of efficient opposed piston engines.

PatOP: Single-Crankshaft Opposed-Piston Engine

http://www.pattakon.com/pattakonPatOP.htm

OPRE: Opposed piston Pulling Rod Engine

http://www.pattakon.com/pattakonOPRE.htm

  > A 500cc two stroke can easily make some 80 Nt*m (8 Kp*m) torque.
  > At 6000 rpm this torque makes some 50 KW (70 PS).
  > 50 KW from 20 Kp means 0.4 Kp per KW.
  > And 0.4 Kp/KW with direct injection Diesel efficiency sounds interesting, especially for an engine with such a low cost."