I have a Sunl-50 and have always thought it had too much power to be a 50, and the cylinder length is the same as my Honda Passport 70. Just noticed that on the Redcat page, the FX-55 is gone, but there's a FX-75 that looks identical and claims a 70cc engine. I'm thinking it was a 70 all along.

 

all those motors are is a honda clone motor. honda had in the past( no more because of compitistion) sold old patents of there motors to the chinesse. what they can do with this motors is (and which is why honda is so great) they made a motor years ago that can go from a 50cc up to a 110cc same motor just a different bore and stroke

and yes honda parts will work on these units