Holes burned through in the center of the piston or light sandblasted look around the perimeter with seizure marks on the skirt or spark plugs backing out after being tightened properly or have broken tip ceramic are caused from detonation.
Detonation is caused after the ignition spark at the plug starts the flame kernel or in the case of severely over heated engines before the spark and as it spreads thru the chamber cylinder pressure rises to the point that the rest of the unburned fuel spontaneously combusts and then the two ignition source combustions collide with each other causing the knock or rattle that you hear (or not hear with loud pipes) as it destroys the piston like a blow torch hammer. During normal combustion, the flame will spread like lighting the corner of a piece of paper and how it consumes it smoothly
What are the causes of detonation?
1-To small of main jet or needles not set correctly. What size were you using and clip position?
2-To high compression. Stock head shaved? What cc dome? What was cranking pressure? It should be 150psi or less to run 94 octane
3-To low of octane fuel for compression used or ignition advance used (4 degrees max for trail and general use and 94 octane at stock compression 130psi up to 150) What fuel do you use?
4-Air leaks into engine from any gasket, crankshaft seals or case parting line, this messes up the normal jetting by adding air after the carb has mixed the fuel and air at the proper ratio. Did you leak test your damaged engine before you disasembled it or after a fresh rebuild?
5-Wrong heat range spark plug causing auto ignition like a glow plug.
6-To tight of squish clearance or to wide of squish band between piston and head for the rpm the engine is trying to run at. As the piston approches the head it pushes the fuel air charge from the perimeter to the center of the bore. The tighter or wider that these critcal dimensions are can cause auto ignition of the fuel charge depending on rpm, bore diameter, squish clearance, squish band width, engine stroke, fuel octane value, air fuel ratio and ignition timing. The higher the rpm the looser and thinner the band needs to be. For low rpm the wider the band can be to make more torque.
7-To much ignition timing advance Go through the list and check off the known to be good ones then work on the unknown issues.
Building an engine for more peak power has its limitations if all the above are not considered as a whole. You can get some more pistons and gaskets and have at it again. We would recommend that the bottom end be disassembled so you can get all particles of piston removed from bearings and case. Install new seals and look at the above items. Mid range burn down is when the throttle is half open and you are just cruising along and you have piston seizure from to lean of fuel to air ratio. This is caused by incorrect main jet size and or the needle clip is to high so that the needle is to low in the needle orifice. This will make it run leaner than it should at throttle positions less than wide open.