Often just reringing the pistons works...you need to set the piston ring end gap in the cylinders...use the piston upside down to get the rings square in the cylinder and find the tightest section of the bore. The ring gap is a factor of pi (3.14159...) times the cylinder bore variation, so if you have a bore taper of say .007" you'll have a ring gap variation of approximately .021", which IMHO is excessive. The rings work too much and will fatigue and crack or simply wear out...blowby may also be an issue. Childs and Albert MAY make a zero-gap piston ring setup (actually a pair or trio of overlapping rings for gap-free operation) for your piston, but you'll have to check. FWIW, they can offer superior sealing over the stock rings.

Another two issues with used pistons are (1) piston to cylinder wall clearance...tighter is better for oil control, worse for high rpm/friction. Case in point, stock Triumph 650 Tiger turns 7500 rpm with one carb at 113mph (1970) and piston/wall clearance of .007"...same engine with new hepolite pistons fitted by #$@%#$ BMW bike shop fitted at .002 lost 1000 rpm and 10mph...but my was it quiet and didn't burn any oil unlike the OEM setup. Also had to break it in for 1000 miles gently whereas the OEM Trump setup was ready as Freddy from the get-go. And (2) ring land to piston ring clearance, less is better, too much and the ring twists too much and can either cause blowby or excessive scraping/wear. Yes there are specific L-ring sections that are designed to twist for improved sealing, but they're not stock to this engine, I believe.

Heck, the more I think about it, replace the #$@%@#$ pistons and be done with it. Also have the conrods sized, honed, etc. and consider replacing the con rod bolts with new high quality ARP bolts. I personally like floating piston pins (gudgeon pins to the UK members!) Use a stretch measurement device for torquing the rod bolts.