Your Rotax 185UL, 277F/277UL, 440F/447UL, 503F/503UL, are Piston Port Engine's, the 462UL, 532UL, 582UL, 618UL, 670, are Rotary Valve Engines. The 277F/277UL, 503F/505UL, and the Skidoo 550F Reed Fed Engine all use the same Head/Cylinder Bolt Pattern. A Reed Fed Engine will outperform a Piston Port Engine. 250(72mm x 61mm) 248.4cc 277(72mm x 66mm) 268.8cc 503(72mm x 61mm) 496.9cc 550(76mm x 61mm) 553.6cc Most Reed Fed Engines the Reed Valve is on the Engine Case. On the Skidoo 550F, it's on the Cylinder. The 550F Cylinder & Case has a Small Port that the 277/277UL, 503F/503UL doesn't have, but you have a choice, add the Small Port on the Case, or Block off the Small Port on the Cylinder and use a Piston with Side Window Port. So what all does this get you. On a 277(72mm x 66mm) 268.8cc, with a 550 Cylinder, Piston & Rod (76mm x 66mm) 299.5cc. Some Minor Port Work needed for the different Stroke. On a 550 Cylinder with a 250 Type 253 61mm Stroke Crank (76mm x 61mm) 276.8cc. On a 503(72mm x 61mm) 496.9cc. With a 503F/503UL Bottom, with 550 Cylinders gets you a (76mm x 61mm) 553.6cc Engine. Stock Cylinders can be Bored 2mm. A 2004 550F with 34mm Carbs, Dynoed 70hp@7000rpm using Stock 9.6cr. So at 6500rpm it should make 65hp. Bump the CR up to 11.6 and gain about 4hp = 69hp! 550F's have been Big Bored to 81mm using an Arctic Cat Piston. (81mm x 61mm) 628.9cc.