If tank is rusty inside the engine will be rusty too. If piston rings and bores have been in same position for long time with wet air, bores must be very rusty in need of a rebore.
0.5mm oversized pistons will not be enough, 0.75-1,0mm might work. I know since I have recently bought a rusty cylinder where 2 sleeves had to be replaced to match the +0.5mm pistons I have for it. I should have bought +1mm pistons. I have a part hording friend that had used stock sleeves on the shelf.
Head job and rebore is my guess if tank has loads of rust inside. Engine hopefully with oil inside, if not the entire engine to be taken apart and clean the rust out. It might be rusty internals anyway that need to be flushed out with some oil changes between running engine until warm.
No rust in tank, spray thin oil in the plug holes and carefully move the crank by the kick starter to feel if stuck.