it's a bit more complicated than that... from
http://www.pinrepair.com/minibikes/tecumsehmb.htm#partnum
Thin and thick ring HS40 pistons are interchangeable. but you probably want to match the piston to the style of rings you have.
HS40 piston/pin (thin rings): #35541
HS40 piston/pin (thin rings): #35542 (.010 over)
HS40 piston/pin (thin rings): #35543 (.020 over)
HS40 piston/pin (thick rings): #32603
HS40 piston/pin (thick rings): #32604 (.010 over)
HS40 piston/pin (thick rings): #32605 (.020 over)
HS40 piston rings (thin rings): #35547
HS40 piston rings (thin rings): #35548 (.010 over)
HS40 piston rings (thin rings): #35549 (.020 over)
HS40 piston rings (thick rings): #34854 (or #33315)
HS40 piston rings (thick rings): #34855 (.010 over)
HS40 piston rings (thick rings): #34856 (.020 over)