Rust
Back in the 70s I pulled a LOT of these engines down. Most which were seized had serious rust in them where water had entered via the carby. They invariably had rust in the two cylinders with open inlet valves. Sometimes what looked a bit sad wasn't too bad, but sometimes the only thing which would move them was to bash the pistons out downwards once the crankshaft had been removed. These motors would sometimes need sleeving, or be scrapped altogether.
The cracks Geoff mentions were very common in the vavle seat area, mostly on the later 8BA engines (in fact it was rare to find one without cracks) but uncommon in earlier motors like the Mercury.
My advice is to try the soaking methods others have mentioned first but be prepared for the worst.
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