yes a 025 piston and cylinder is a 42.5mm.
You can get the piston rings and seals on e-bay for a little of nothing. i just found this on there
Piston Crankshaft Crank shaft Bearing Oil Seal For STIHL CHAINSAW 025 MS250 NEW | eBay
There are 2 different sizes for 025 I thought... 42mm and 42.5mm. at least northwoods shows both. I would put a caliper/micrometer on the top of the piston to measure it's diameter or measure the cylinder if possible.
and no offense, but i'd get a good A.M. piston and rings from northwoods for $25-$33
Quality replacement pistons from Meteor and Episan