Yup...that would be the culprit right there.

If everything else looks good inside the engine, you should be good to go once you replace that part. While you're at it, replace the con-rod and the two locking springs (never re-use these)

I don't mean to sound like you don't know what you're doing, but I've made this simple mistake myself and destroyed a piston. Make sure you put the piston back in the right way round. You should see that one side of the lower piston edge has a curved 'cut out' section, this is to allow clearance between the piston and crank shaft at the bottom stroke.

I once re-assembled a costly OS without ensuring I did this correctly. Needless to say.....1 shattered piston with large bits-n-pieces flying around the inside of the case....[:@]

Good luck