I have experience with both the K&b 3.5 and the Novarossi (side exhaust) 09/20c. The Novarossi is a stronger engine but also heavier. I could not get a 10 sized shrike to balance (even with a AA battery pack all the way in the back) with the NR. The K&B is lighter and I can get 16.2K on a MA 8x6 on a tuned pipe and 10% Omega fuel. Considering my lack of patience with tuning a tuned pipe you could probably get more RPM. Using the stock muffler I get 14.7K on the MA 8X6 and 19.3K on a MA 8x3.
I vaguely remember the NR bettering the K&b by about a 1000 rpm. I had one in a HOB Mustang and it seemed to like a 7x6W prop. It was a good engine (heavy) but I didnt really have a use for it after I sold the Mustang.
FWIW I own 4 of the K&B 3.5's. One is on a Sig Wonder. One that is about worn out (where the rpm numbers listed earlier came from) is on a PBF, One is destined for a Shrike (Hopefully one that will not fold its wings again) and I will find a use for the other one. The 21 sized speed planes Hobby Barn is selling looks promising.
Its a shame K&B doesnt make the 3.5 in an aero version any more. I really do like this engine. Maybe you can get some useful info from this post
