It's pretty simple to do, but with RC designed engines selling at the prices they do now, it's barely worth the effort unless you just want to. In a nutshell, you make a new exhaust that flows more and redirects the heat down and out of your cowl (pitts type muffler is typical), you cut off anything not needed to mount the engine to the airframe, and you make or buy a prop adapter. It's optional to remove the flywheel and magneto to set up a CDI ignition, but if you do you'l save weight and get more horsepower and reliability.