I doubt you have rust in the cyl..the cyl is chrome or nikasil lined and won't rust...the rings are cast iron and can rust but there is nothing for them to rust to. Can you turn it over with a wrench?? If you can...pull the muffler and rotate the crank so the piston is all the way down..you should be able to see in the exhaust port and spark plug hole. With a flash light shining in one look in the other and you should be able to see a good percentage of the bore, some from each location.
I got a 630 awhile back that the seller said was locked up...and it was!! It had a brand new never fired spark plug in finger tight and when I looked in the hole there was a lot of fuzz and crap in the cyl but the piston was nearly all the way up. I filled it up with wd 40 and let it sit overnight. The next day I was able to turn the crank with wrench and much difficulty, about half a revolution. I pulled the four cyl bolts and the cyl slid right off!!! The piston was fine the bore was fine but the main bearings were froze and real rusty!! I put the whole thing in a box...I was wasn't to depressed as I bought it just for parts and was not expecting much...But I wasn't expecting that!!!
I would make sure your problem is in the P&C and not the lower end before I went any further. If you aren't satisified with what you see pull the cyl and get a good look. Easier to do it now than after it's blown up...