The Nova Rossi is a screamer, I had one for a while. You'd have to do a lot of modifying to get it in your boat though, the downside. It might be easier to go up to a .45 than to go down to a .21 .
The Nova Rossi would be in my top 3 picks for a .21 boat though. The other 2 would be a MAC and the Rossi Niagara/Pirahna that I own now. The Rossi is kind of a slug until it gets about a gallon of fuel through it, every run with that motor was faster than before. John Finch did a review of it in RCBM but I don't think that he really put any serious run time on it. Mine flat gets it, a MAC killer!