I ran the OS 15 cv-r(x) in my xxx-nt (rtr) when I had it. Pretty quick engine. I'd gear it to the roof , drop the suspension and throw some pro-like road hawgs on it for parking lot use. If it wasn't set up for that, I ran 1 tooth smaller than the RTR came with with losi step pins in teh rear (best sticky tire out there) with whatever fronts that came on it. That thing would scream.
Side note, I ran 6 gallons through that engine, then traded the truck. New owner has put 1.5 (probably 2 by now) in the same engine and it still runs like a top.