Sounds a bit like one disaster after another.
Not sure about putting a piston from one engine into another, if the original piston is only lightly marked where the ring popped out ( I am guessing the scratch is from the bottom ring down towards the base or skirt of the piston) I would polish it with a very, very fine bit of wet and dry to remove the scratch and polish it with Solvo polish, as long as the ring groove is not compromised. The bottom rings are not usually compression rings but oil control rings, or oiler rings, so will not impact on engine compression. Gently pop on the oiler ring from one of the pistons you have bought and use that, saves removing the Gudgeon Pin from the original piston and the hassle of replacing the gudgeon pim circlips.
The skirt or land of the piston having a scratch or mark with not effect the performance of the engine as it is the rings that are in mechanical contact with the bore not the alloy piston itself.
If the scratch is really deep, wide and long then replace the piston as it could compromise the strength of the piston at high revs causing the piston to break up or fail in some way.
If you do decide to remove the piston, stuff the crankcase mouth with rags before you start, because a gudgeon pin circlip falling into the crankcase mouth will cause a whole new set of problems.