A 2004 550F uses 9.6cr, Twin 34mm Carbs, Dynoed 70hp@7000rpm! Later 550F's come with Smaller 30mm Carbs, and later ones even have Smaller Port Cylinders, rated 58hp@6800rpm. A 550F (76mm x 61mm) 553.6cc.
A 2 Stroke using 11.5cr and turned 6500rpm, Carbed right, with a Good Tuned Pipe it takes on Avg 7cc to make 1hp. A +/- 1.0cr = Avg 1-1.5hp. So 553cc/7cc = 79hp. A 503F is 496.9cc/7cc = 70.9hp.
A 503UL, same Core Engine as a 503F, they use 10.8cr and 36mm Carbs and uses a Muffler, not a Tuned Pipe, and is rated 50hp@6500rpm. The Sled 503F uses 34mm Carbs and was rated around 58hp@7000rpm. A 503UL with an R&D Aero Tuned Pipe made 62.3hp@6500rpm, and that's a very Mild Tuned Pipe! 50hp + 22.5% = 62.5hp.
A 521, 583 use 38mm Carbs, so a 550F could also use a 38mm Carb. More Air = more hp.
Cylinder & Case Porting can add 10-15% more hp.
So to answer your question.
1. For Maximum hp Gain use an After Market Tuned Pipe.
2. Use Bigger carbs.
3. Do Cylinder & Case Porting.
4. Raise the CR.
5. Do all of these plus a Big Bore. Max Bore with a Custom Sleeve is 82mm. The 277F, 503F, 550F, and 521 to 670 All use a 4 or 6 Bolt Pattern based on a 100mm Bolt Circle. The 670 has been Big Bored to 82.5mm.
277F uses 11.8cr
377F uses 9.6cr
440F uses 9.6cr
503F uses 10.8cr
All Water Cooled 470 to 670 use 11.5cr
670HO uses 12.5cr.