Motor slows down

Hello, so as already mentioned opener my 1.8 PM after 337,000 running killometers makes slowly but surely sluggish. For me also no problem further as such things do not evade my professional knowledge. My question is now whether it would be cheaper to have the piston rings re-made (as these are so from the bottom of the line you don’t believe) or whether it is cheaper to shoot a new engine at Ebay oä.? Thanks already in advance Mfg Julius