I need a bore gauge now? Lol
What is a bore?
What is the port?
What parts is it missing?
I agree - that cylinder needs attention. The vertical scratches are why you're losing compression. Also, as Scott said, you're missing part of the piston, possibly due do detonation at some point in the engine's past.
You do not need to buy a bore gauge - you need to disassemble the engine an take the block to a competent machine shop and have them do the work. They will have the bore gauge and other necessary equipment to inspect and repair the block. They will measure the bore (diameter) of the cylinders and if there is enough wall thickness in the cylinders, they can either simply hone out the damage, or bore then hone.
If it only required a hone, you may be able to get away with re-using 2 of your 3 pistons and just re-ring them. If your block requires a bore and hone, you'll have to replace all 3 pistons to go up to the next size. Either way, you'll want to replace the piston with the missing material.
In the pictures below, I've drawn arrows to show you the ports (holes in the cylinder walls - present on 2-stroke engines), and the areas where you are missing part of your piston.