My suggestion would be to take the cylinder off, and block one side. Fill the water jacket with Evapo-rust. I’d give it a few days at least. It would be helpful if you could get the cyl. up to around 100 degrees. If the level does drop, that would indicate you have broken through the blockage. Fill the cylinder again and give the Evapo-rust a few more days to dissolve the rust in the blocked area. It may not drop if the Evapo-rust is working from both sides. If you can’t get through the blockage you may need to remove the core plugs..
A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.
