I have an OS .21 SE-M in an old Graupner Arrow mono hull boat. It runs decent, but is no powerhouse. My main beef with these OS engines is the poorly manufactured Nickel liners. If I had to choose between a Rossi and an OS, I'd take the Rossi every time. A far better engine. I will be swapping my OS out for a Novarossi when my hobby budget permits it. I would try this engine:
http://www.novarossidirect.com/RPLUS213FT_p_3275.html