You usually check the piston to bore fit by measuring the sizes of both with micrometers. However provided both bore and piston are not scored or scuffed, you can check the fit by seeing if the piston can be moved sideways in the bore. If it moves noticeably, it is pretty loose - which messes up the port sealing (and makes a nasty noise, usually).
You can most likely put Victa rings into the bore just by pinching them with your fingers. If not, use a piece of thin aluminium or steel sheet wrapped around the piston for exactly one turn, and compress the rings with tie-wire around the outside of the sheet. Remember the rings are kept from rotating in their grooves by brass pegs screwed into the piston. Do not try to remove those pegs - they are permanently installed. They are needed because they keep the rings from moving around in the ring grooves, and crossing the ports. If ring gaps cross a port they may score the bore and/or break/chip the ends of the rings.
The seals we referred to are the air/oil seals on the top and bottom of the crankshaft, beside the main bearings. If they leak air during the crankshaft suction phase, you get lean mixture. To test them, set the engine up with the crankshaft vertical (and the flywheel removed), and trickle a little bit of fuel (not oil) onto the seal. If it disappears, the seal leaks and has to be replaced. Turn the engine the other way up and repeat for the other seal.
