Is the clutch hub and basket lined up correctly seem to remember you can fit the dt one 4 different ways but will only work in one position

Hard to tell in the pic but looks like the gudgeon pin circlip is missing? Unless you took it out before pic.

Looks like your missing a bolt from the crank cases near inlet though.

Pretty sure there different where the oil pump mounts no?

From experience it tends to be running too lean that kills two strokes. Though a 2 stroke makes more power the leaner it runs. At 57mm you often have to modify the power valve for clearance. And the head gasket will need opening up. When the barrel 1st seized they might have replaced it with a 'good' second hand barrel that had already been taken out a few sizes.

@gilerasc125 at 57mm bore it could well be catching powervalve and/or head gasket. I'd whip the powervalve out and check for any witness marks.

Piston rings have a hard life in a 2 stroke travelling over the open ports. I've seen the pins that stop the rings rotating on the piston move, allowing the ring to spin and open end catch on a port. With it being under load at time it failed (going up a steep hill), I wouldn't be surprised if piston expanded due to heat build up and pinched the rings. May have been running on the lean side aswell.

When you say it has compression is that with a gauge or just thumb over plug hole? A mates bike had very similar symptoms, could feel compression with your thumb, gauge only showed 25psi though. On taking barrel off piston rings were in pieces and barrel heavily scored. Amazed it even had 25psi!