If you go to a skinnier cartridge, you will have feeding issues.
The cartridge will pop up prematurely, this may not matter on a push feed too much, but it will be an issue.
If it were me, I would spin up some scotch brite on a dowel and polish out the rust and give the throat an inspection, it may be superficial and not a concern.
You could sell it as a barreled action and keep the stock.
Cheers.
