Hi tinu, just reading your post again and again to make some sense out of it.
Looks as if the bike has done some 5K Km with no problems on plus 20 flat top pistons with a compression plate on the cylinder base. Then you mention plus 40's. Does this mean you have had it rebored to plus 40, but need to be sure the oil supply is OK?
If the plus 20's seized soon after being put in, then the cause is most likely a too tight bore/piston clearance. If it suddenly happened, after some uneventful regular use, then something is wrong.
If you still have the barrel on plus 20, get the bore honed, clean up the pistons, make sure the rings are free and try again. It may rattle a bit, but you know why, and it is worth doing before spending again. Of course the perfectionists will be in horror, but they are not spending the money.
Without stripping the motor, the oil pump output can be checked by removing the pressure relief valve....you should get a good dribble of oil when you rotate the crankshaft. You can also check for any lift in the big ends, as an indication of any bearing problem due to lack of oil.
A couple of additional posts were added while I was typing this. Bent rods are unlikely if the piston damage is more or less consistent on both pistons. Straight rod across the small end bushes, as suggested. Heating pistons with a hot air gun means the pins will push out with very little force.
Piston manufactures state the nominal bore, plus the oversize, and this is the size the finished, honed bore should be. The piston is manufactured to a size which takes into account this running clearance. Some machinists discard this information, measure the piston and add on a clearance from their experience, usually with water cooled engines which run to a tighter bore than air cooled motors. Some are not too happy to be told they got it wrong, so folks don't complain and just go somewhere else, so the guy continues to bugger pistons and bores for years, unaware of the misery he has caused.
Clearance is the diameter of the piston measured at the pin height, at 90 degrees to the pin, compared to the bore diameter. Pistons are slightly oval, being narrower each side, compared to thrust faces, so the measurement is the thrust faces, front to back, not the pin end sides. The piston crown above the top ring is narrower than the skirt, which is why you can sometimes see the top ring when you look down the bore and foolishly convince yourself it's worn out and needs a rebore! The piston running clearance is in the BSA published data for your model year.
Swarfy.