Paul, there are big bore kits offered for your Evolution motor. The kits usually require that your crank cases be honed to accept the larger cylinders. That makes the job a bit more involved than your buddy’s Twin Cam kit.
S&S offers their classic Sidewinder package. Check that out. Here’s a link:
A big bore kit is not the only way to gain significant horsepower.
Consider Screamin Eagle heads with a more aggressive cam.
A Thunderheader, or Supertrapp exhaust also aids a performance package/combination.
Choose a cam that reflects your riding style.
The Evo has been around for three decades now. People have figured out a lot of different combinations for creating more horsepower. Do your homework. Search on line to see what others have done. Read reviews of kits & combinations that have been tried. Find something that fits your budget.
Don’t rule out Nitrous. That is a lot of fun too!