Hopefully you changed the ROD

Did u try lightly tapping on the ring to FREE it up

if its rusted than the pistin is pitted  and NO GOOD

I hope they are really close to the same weight or your going to have
some real vibration issues. If that piston is on the heavy side it will
probably take out the crank in short order.
That ring is NOT rusted in, it is packed with carbon and that is causing
it to stick in the piston. If the rings are that bad I hope you also
replaced them all, If not don't bother finishing that engine, it will
drink oil and probably have such bad compression it may not even start.

To get that ring out there is an easy way. Find the ring gap, then use a
small punch and tap on one side of the gap, you want to push one side in
just enough to get at the tip of the other side. Then tap on the other
side like you are trying to spin the ring, this should either break the
ring loose OR break the ring itself. Then just keep breaking out pieces
till the ring groove is open, then use a small piece of the broken ring
to scrape the rest of the crud and carbon out of the groove. DO NOT
REMOVE METAL FROM THE PISTON. Just remove the crud. Clean ALL the ring
grooves like this and then wash down the pistons with clean solvent and
install the new rings.